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Document and Entity Information	12 Months Ended
Document and Entity Information	Dec. 31, 2017 shares
Document And Entity Information [Abstract]
Document Type	20-F
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	NCNA
Entity Registrant Name	NUCANA PLC
Entity Central Index Key	0001709626
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding	31,811,146

Consolidated Statements of Operations - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Research and development expenses	£ (17,673)	£ (7,904)	£ (5,655)
Administrative expenses	(4,573)	(1,143)	(1,251)
Initial public offering related expenses	(1,794)
Net foreign exchange (losses) gains	(1,654)	599	(8)
Operating loss	(25,694)	(8,448)	(6,914)
Finance income	208	283	406
Loss before tax	(25,486)	(8,165)	(6,508)
Income tax credit	2,401	2,116	1,176
Loss for the year	£ (23,085)	£ (6,049)	£ (5,332)
Basic and diluted loss per share	£ (0.89)	£ (0.25)	£ (0.22)

Consolidated Statements of Comprehensive Loss - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Loss for the year	£ (23,085)	£ (6,049)	£ (5,332)
Items that may be reclassified subsequently to profit or loss:
Exchange differences on translation of foreign operations	(8)	(2)	(1)
Other comprehensive expense for the year	(8)	(2)	(1)
Total comprehensive loss for the year	(23,093)	(6,051)	(5,333)
Attributable to:
Equity holders of the Company	£ (23,093)	£ (6,051)	£ (5,333)

- Definition

The total of income less expenses, excluding the components of other comprehensive income. [Refer: Other comprehensive income]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/exampleRef
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+ Details

Consolidated Statements of Financial Position - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Non-current assets
Intangible assets	£ 1,938	£ 1,377
Property, plant and equipment	358	18
Deferred tax asset	81
Total non-current assets	2,377	1,395
Current assets
Prepayments, accrued income and other receivables	3,050	3,634
Current income tax receivable	4,225	2,195
Cash and cash equivalents	86,703	19,990
Total current assets	93,978	25,819
Total assets	96,355	27,214
Capital and reserves
Share capital and share premium	80,508	43,433
Other reserves	58,071	4,064
Accumulated deficit	(45,159)	(22,256)
Total equity attributable to equity holders	93,420	25,241
Non-current liabilities
Provisions	18
Current liabilities
Trade payables	1,120	728
Payroll taxes and social security	157	61
Accrued expenditure	1,640	1,184
Total current liabilities	2,917	1,973
Total liabilities	2,935	1,973
Total equity and liabilities	£ 96,355	£ 27,214

Consolidated Statements Of Changes In Equity - GBP (£) £ in Thousands	Total	Share Capital	Share Capital Series B Shareholders	Share Premium	Share Premium Series B Shareholders	Own Share Reserve	Share Option Reserve	Foreign Currency Translation Reserve	Capital Reserve	Accumulated Deficit
Beginning balance at Dec. 31, 2014	£ 34,508	£ 659		£ 42,574		£ (339)	£ 2,506			£ (10,892)
Loss for the year	(5,332)									(5,332)
Other comprehensive expense for the year	(1)							£ (1)
Total comprehensive loss for the year	(5,333)							(1)		(5,332)
Share-based payments	785						785
Ending balance at Dec. 31, 2015	29,960	659		42,574		(339)	3,291	(1)		(16,224)
Loss for the year	(6,049)									(6,049)
Other comprehensive expense for the year	(2)							(2)
Total comprehensive loss for the year	(6,051)							(2)		(6,049)
Share-based payments	1,132						1,132
Exercise of share options	200	4		196			(17)			17
Issue of share capital		4		196
Ending balance at Dec. 31, 2016	25,241	663		42,770		(339)	4,406	(3)		(22,256)
Loss for the year	(23,085)									(23,085)
Other comprehensive expense for the year	(8)							(8)
Total comprehensive loss for the year	(23,093)							(8)		(23,085)
Share-based payments	11,731						11,731
Reduction in share premium				(42,466)					£ 42,466
Exercise of share options	120	1		119			(180)			180
Lapse Of Share Options							(2)			2
Bonus issue to series B shareholders		304	£ 304	(304)	£ (304)
Issue of share capital	79,834	304		79,530
Transaction costs	(413)			(413)
Ending balance at Dec. 31, 2017	£ 93,420	£ 1,272		£ 79,236		£ (339)	£ 15,955	£ (11)	£ 42,466	£ (45,159)

Consolidated Statements of Cash Flows - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Loss for the year	£ (23,085)	£ (6,049)	£ (5,332)
Adjustments for:
Income tax credit	(2,401)	(2,116)	(1,176)
Amortization and depreciation	194	101	44
Finance income	(208)	(283)	(406)
Share-based payments	11,731	1,132	785
Initial public offering (IPO) related expenses	1,794
Net foreign exchange losses	1,584
Cash flows from (used in) operations before changes in working capital	(10,391)	(7,215)	(6,085)
Movements in working capital:
Decrease (increase) in prepayments, accrued income and other receivables	458	(3,404)	36
Increase in trade payables	392	220	133
Increase in payroll taxes, social security and accrued expenditure	551	33	418
Movements in working capital	1,401	(3,151)	587
Cash used in operations	(8,990)	(10,366)	(5,498)
Corporation tax	282	1,102	1,031
Net cash used in operating activities	(8,708)	(9,264)	(4,467)
Cash flows from investing activities
Interest received	162	410	290
Payments for office and computer equipment	(370)	(15)	(6)
Payments for intangible assets	(725)	(539)	(390)
Proceeds from (investment in) short-term deposits		15,075	(75)
Net cash (used in) provided by investing activities	(933)	14,931	(181)
Cash flows from financing activities
Proceeds from issue of share capital	79,834
IPO related expenses from issue of share capital - included in share premium	(413)
IPO related expenses included in statement of operations	(1,794)
Proceeds from issue of share capital - exercise of share options	120	200
Net cash from financing activities	77,747	200
Net increase (decrease) in cash and cash equivalents	68,106	5,867	(4,648)
Cash and cash equivalents at beginning of year	19,990	14,112	18,761
Foreign currency translation differences	(1,393)	11	(1)
Cash and cash equivalents at end of year	£ 86,703	£ 19,990	£ 14,112

General Information	12 Months Ended
General Information	Dec. 31, 2017
Disclosure Of General Information About Financial Statements [Abstract]
General information	1. General information NuCana plc (“NuCana” or the “Company”) is a clinical-stage biopharmaceutical company developing a portfolio of new medicines to treat cancer. We are harnessing the power of phosphoramidate chemistry to generate new medicines called ProTides. These compounds have the potential to improve cancer treatment by enhancing the efficacy and safety of several current standards of care. On August 29, 2017 the Company re-registered as a public limited company and changed its name from NuCana BioMed Limited to NuCana plc. The Company has had American Depository Receipts (“ADRs”) registered with the US Securities and Exchange Commission (“SEC”) and is listed on Nasdaq since October 2, 2017. The Company is incorporated in England and Wales and domiciled in the United Kingdom. The Company has two wholly owned subsidiaries, NuCana, Inc. and NuCana BioMed Trustee Company Limited. The financial statements for the Company were authorized for issue by the board of directors on March 21, 2018.

Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Disclosure Of Summary Of Significant Accounting Policies [Abstract]

Significant accounting policies

2. Significant accounting policies

Basis of preparation

The financial statements have been prepared in accordance with International Financial Reporting Standards (“IFRS”) as issued by the International Accounting Standards Board (“IASB”).

The Company’s financial statements comprise the financial statements of the Company and its subsidiaries at December 31, 2017. The financial statements are presented in pounds sterling, which is also the Company’s functional currency. All values are rounded to the nearest thousand, except where otherwise indicated.

Going concern

In common with many companies in the biopharmaceutical sector, the Company incurs significant expenditure in its early years as it researches and develops its potential products for market.

The board of directors, having reviewed the operating budgets and development plans, considers that the Company has adequate resources to continue in operation for the foreseeable future. The board of directors is therefore satisfied that it is appropriate to adopt the going concern basis of accounting in preparing the financial statements. The Company believes that its cash and cash equivalents of £86.7 million at December 31, 2017, will be sufficient to fund its current operating plan for at least the next 12 months. As the Company continues to incur losses, the transition to profitability is dependent upon the successful development, approval and commercialization of its product candidates and achieving a level of revenues adequate to support its cost structure. The Company may never achieve profitability, and unless and until it does, it will continue to need to raise additional capital. There can be no assurances, however, that additional funding will be available on acceptable terms.

2. Significant accounting policies (continued)

Judgements and estimates

The preparation of the financial statements requires management to make judgements, estimates and assumptions that affect the amounts reported for assets and liabilities at the balance sheet dates and the amounts reported for revenue and expenses during the year. The nature of estimations means that actual outcomes could differ from those estimates.

The following judgements have had the most significant effect on the amounts recognized in the financial statements:

Research and development expenses

The Company recognizes research and development expenses in the statement of operations in the period in which they are incurred. When development activities reach the advanced stage, as set out in the specific criteria of International Accounting Standard (“IAS”) 38, Intangible Assets, there will be a requirement to capitalize such costs as intangible assets. Management will continue to exercise judgement in the appropriate treatment of development costs.

Taxation

Management judgement is required to determine the amount of deferred tax assets that should be recognized, based upon the likely timing and level of future taxable profits. Further details are contained in Note 7.

The following estimates have had the most significant effect on the amounts recognized in the financial statements:

Recognition of Clinical Trial Expenses

As part of the process of preparing our consolidated financial statements, we may be required to estimate accrued expenses related to our clinical trials. In order to obtain reasonable estimates, we review open contracts and purchase orders. In addition, we communicate with applicable personnel in order to identify services that have been performed, but for which we have not yet been invoiced. In most cases, our vendors provide us with monthly invoices in arrears for services performed. We confirm our estimates with these vendors and make adjustments as needed, however, our estimates are dependent on the completeness of information from our vendors. The following are examples of our accrued expenses:

•

fees paid to Clinical Research Organizations (CROs) for services performed on preclinical studies and clinical trials; and

•

fees paid for professional services.

Share-based payments

Estimating fair value for share-based payment transactions requires determination of the most appropriate valuation model, which depends on the terms and conditions of the grant. This estimate also requires determination of the most appropriate inputs to the valuation model, including the expected life of the share option or appreciation right, volatility, dividend yield and assumptions about them and, in the case of the Company, the value of an ordinary share. For the measurement of the fair value of equity-settled transactions with employees at the grant date, the Company uses the Black-Scholes model. The assumptions and models used for estimating fair value for share-based payment transactions are disclosed in Note 13.

Basis of consolidation

The consolidated financial statements comprise the financial statements of the Company and its subsidiaries.

Subsidiaries are consolidated from the date of acquisition, being the date on which the Company obtains control, and continue to be consolidated until the date when such control ceases. The financial statements of the subsidiaries are prepared for the same reporting period as the parent company, using consistent accounting policies. All intra-group balances, transactions, unrealized gains and losses resulting from intra-group transactions and dividends are eliminated in full.

2. Significant accounting policies (continued)

Assets, liabilities, income and expenses of a subsidiary acquired or disposed of during the year are included in the consolidated financial statements from the date the Company gains control until the date the Company ceases to control the subsidiary.

Segment reporting

The Company operates in one operating segment. The Company’s chief operating decision maker (the “CODM”), its Chief Executive Officer, manages the Company’s operations on an integrated basis for the purposes of allocating resources. The Company’s principal operations and decision-making functions are located in the United Kingdom. The CODM makes decisions on a global basis from there. Accordingly, the Company has determined that it operates in a single reporting segment.

Initial public offering (IPO) related expenses

Incremental costs incurred and directly attributable to the offering of securities were deducted from the related proceeds of the offering. The net amount is recorded as contributed shareholders’ equity in the period when such shares were issued. Costs that relate to the stock market listing or are otherwise not incremental and directly attributable to issuing new shares, are recorded as an expense in the statement of comprehensive income. Costs that relate to both share issuance and listing are allocated between those functions on a rational and consistent basis. In the absence of a more specific basis for apportionment, an allocation of common costs based on the proportion of new shares issued to the total number of (new and existing) shares listed has been used.

Property, plant and equipment

Property, plant and equipment is stated at cost, net of accumulated depreciation and accumulated impairment losses, if any. There are no restrictions on title or equipment pledged as security for liabilities.

Depreciation is provided on property, plant and equipment over their expected useful economic life as follows:

Asset class

Depreciation method and period

Office and computer equipment

Fixtures and fittings

Straight-line over 3 years

Straight-line over 5 years

Intangible assets

Intangible assets are stated at cost, net of accumulated amortization and accumulated impairment losses, if any. Cost in relation to patents includes registration, documentation and other legal fees associated with obtaining the patent. Software costs represent the initial purchase price of the asset.

The amortization method and amortization period for the principal categories of intangible assets are follows:


Asset class		Amortization method and period
Patents		Straight-line over 20 years
Computer software		Straight-line between 3 and 5 years

The Company’s primary patents each have a life of 20 years. Further patents are granted in various jurisdictions to extend the territorial coverage of the primary patent. These patents are granted up to the period of the related primary patent. Costs are thus amortized over the remaining life of the relevant primary patent. The amortization method and the amortization period for an intangible asset with a finite useful life are reviewed at least at each financial year-end. Changes in the expected useful economic life or the expected pattern of consumption of future economic benefits embodied in the asset are accounted for by changing the amortization period or method, as appropriate. The Company changed the amortization method of patents from reverse sum of digits to straight-line from July 1, 2017. These changes are made on a prospective basis and treated as changes in accounting estimates. The effect of this change on the current year is immaterial.

Intangible assets are tested for impairment when there is an indicator of impairment.

2. Significant accounting policies (continued)

Cash, cash equivalents and short-term deposits

Cash and cash equivalents in the statement of financial position include cash at banks with a maturity of less than three months, which is subject to an insignificant risk of changes in value.

Short-term deposits represent certificates of deposits with banks with maturities of greater than three months but less than one year.

Research and development

Research and development expenses are currently recognized in the statement of operations in the year in which they are incurred. Development expenditures on an individual project will be recognized as an intangible asset when the Company can demonstrate:

•

the technical feasibility of completing the intangible asset so that the asset will be available for use or sale;

•

its intention to complete and its ability and intention to use or sell the asset;

•

how the asset will generate future economic benefits;

•

the availability of resources to complete the asset; and

•

the ability to measure reliably the expenditure during development.

Income Taxes

Current income tax

Current income tax assets and liabilities are measured at the amount expected to be recovered from or paid to the taxation authorities. The tax rates and tax laws used to compute the amount are those that are enacted or substantively enacted at the reporting date in the countries where the Company operates and generates taxable income.

Deferred income tax

Deferred income tax is provided in full, using the liability method, on temporary differences arising between the tax bases of assets and liabilities and their carrying amounts in the Company’s financial statements. However, the deferred income tax is not accounted for if it arises from initial recognition of an asset or liability in a transaction other than a business combination that at the time of the transaction affects neither accounting nor taxable profit or loss. Deferred income tax is determined using tax rates and laws that have been enacted or substantially enacted by the balance sheet date and are expected to apply when the related deferred income tax asset is realized or the deferred tax liability is settled. Deferred tax assets are recognized to the extent that it is probable that future taxable profit will be available against which the temporary differences can be utilized.

Income tax credit

The Company benefits from the U.K. research and development tax credit regime whereby a portion of the Company’s losses can be surrendered for a cash rebate of up to 33.35% of eligible expenditures. Such credits are accounted for within the tax provision, in the year in which the expenditures were incurred.

2. Significant accounting policies (continued)

Operating leases

Leases where the lessor retains substantially all the risks and benefits of ownership of the asset are classified as operating leases and rentals payable are charged in the consolidated statement of operations on a straight-line basis over the lease term.

Impairment of non-financial assets

The Company assesses, at each reporting date, whether there is an indication that an asset may be impaired. If any indication exists, the Company estimates the asset’s recoverable amount.

An impairment loss is recognized whenever the carrying amount of an asset or its cash-generating unit exceeds its recoverable amount. Impairment losses are recognized in the consolidated statements of operations.

A cash-generating unit is the smallest identifiable group of assets that generates cash inflows that are largely independent of the cash inflows from other assets or groups of assets.

Calculation of recoverable amount

The recoverable amount of assets and cash-generating units is the higher of their fair value less costs to sell and value in use. In assessing value in use, the estimated future cash flows are discounted to their present value using a pre-tax discount rate that reflects current market assessments of the time value of money and the risks specific to the asset. For an asset that does not generate largely independent cash inflows, the recoverable amount is determined for the cash-generating unit to which the asset belongs.

Reversal of impairment

An assessment is made at each reporting date as to whether there is an indication that a previously recognized impairment loss may no longer exist or may have decreased. If such an indication exists, the recoverable amount is estimated.

A previously recognized impairment loss is reversed only if there has been a change in the estimates used to determine the recoverable amount since the last impairment loss was recognized. If that is the case, the carrying value is increased to its recoverable amount. An impairment loss is reversed only to the extent that the asset’s carrying amount does not exceed the carrying amount that would have been determined, net of depreciation or amortization, if no impairment loss had been recognized.

Share-based payments

Employees, directors and consultants of the Company receive remuneration in the form of share-based payments, whereby individuals render services as consideration for equity instruments (share options).

Under IFRS 2 Share-based Payment, equity share-based payments are measured at the fair value of the equity instruments at the grant date. Details regarding the determination of fair value of equity settled share-based transactions are set out in Note 13.

The fair value determined at the grant date of equity settled share-based payments is expensed on a straight line basis over the vesting period, with a corresponding increase in equity to the share option reserve.

2. Significant accounting policies (continued)

Fair value measurement

Fair value is based on the price that would be received from the sale of an asset or that would be paid to transfer a liability in an orderly transaction between market participants at the measurement date. In order to increase consistency and comparability in fair value measurements, IFRS 13 establishes a fair value hierarchy that prioritizes observable and unobservable inputs used to measure fair value into three broad levels, which are described as follows:

Level 1: Quoted (unadjusted) prices in active markets for identical assets or liabilities.

Level 2: Other techniques for which all inputs that have a significant effect on the recorded fair value are observable, either directly or indirectly.

Level 3: Techniques that use inputs that have a significant effect on the recorded fair value that are not based on observable market data.

Accounting Standards

In preparing these financial statements, the Company has applied all relevant IAS, IFRS and International Financial Reporting Interpretations Committee (“IFRIC”) Interpretations as of the date of approval of these financial statements and which are mandatory for the financial year ended December 31, 2017.

The following accounting standards and interpretations have been adopted as of January 1, 2017 in these financial statements and have not had a material impact on the Company’s accounts in the period of initial application:

Amendments to IAS 7 Statement of Cash Flows: Disclosure Initiative

The amendments require entities to provide disclosure of changes in their liabilities arising from financing activities, including both changes arising from cash flows and non-cash changes (such as foreign exchange gains or losses). The Company has no liabilities arising from financing activities therefore no additional disclosure is required.

Amendments to IAS 12 Income Taxes: Recognition of Deferred Tax Assets for Unrealised Losses

The amendments clarify that an entity needs to consider whether tax law restricts the sources of taxable profits against which it may take deductions on the reversal of deductible temporary differences related to unrealised losses. Furthermore, the amendments provide guidance on how an entity should determine future taxable profits and explain the circumstances in which taxable profit may include the recovery of some assets for more than their carrying amount. The Company applied amendments retrospectively. However, their application has no effect on the Company’s financial position and performance as the Company has no deductible temporary differences or assets that are in the scope of the amendments.

Annual Improvements 2014-2016 Cycle

These improvements include:

IFRS 12 Disclosure of Interests in Other Entities: Clarification of the scope of disclosure requirements in IFRS 12

The amendments clarify that the disclosure requirements in IFRS 12, other than those in paragraphs B10–B16, apply to an entity’s interest in a subsidiary, a joint venture or an associate (or a portion of its interest in a joint venture or an associate) that is classified (or included in a disposal group that is classified) as held for sale. This amendment is not expected to have a material impact on the Company.

2. Significant accounting policies (continued)

The IASB and IFRIC have issued the following standards and interpretations, which are considered relevant to the Company, with an effective date after the date of these financial statements.

IFRS 9: Financial Instruments

In July 2014, the IASB issued the final version of IFRS 9 Financial Instruments that replaces IAS 39 Financial Instruments: Recognition and Measurement and all previous versions of IFRS 9. IFRS 9 brings together all three aspects of the accounting for financial instruments project: classification and measurement, impairment and hedge accounting. IFRS 9 is effective for annual periods beginning on or after January 1, 2018, with early application permitted. Except for hedge accounting, retrospective application is required but providing comparative information is not compulsory. For hedge accounting, the requirements are generally applied prospectively, with some limited exceptions. The Company expects no material impact on its balance sheet and equity.

IFRS 16: Leases

IFRS 16 was issued in January 2016 and it replaces IAS 17 Leases, IFRIC 4 Determining Whether an Arrangement Contains a Lease, SIC-15 Operating Leases-Incentives and SIC-27 Evaluating the Substance of Transactions Involving the Legal Form of a Lease. IFRS 16 sets out the principles for the recognition, measurement, presentation and disclosure of leases and requires lessees to account for all leases under a single on-balance sheet model similar to the accounting for finance leases under IAS 17. The standard includes two recognition exemptions for lessees—leases of ’low-value’ assets (e.g., personal computers) and short-term leases (i.e., leases with a lease term of 12 months or less). At the commencement date of a lease, a lessee will recognize a liability to make lease payments (i.e., the lease liability) and an asset representing the right to use the underlying asset during the lease term (i.e., the right-of-use asset). Lessees will be required to separately recognize the interest expense on the lease liability and the depreciation expense on the right-of-use asset. Lessees will also be required to remeasure the lease liability upon the occurrence of certain events (e.g., a change in the lease term, a change in future lease payments resulting from a change in an index or rate used to determine those payments). The lessee will generally recognize the amount of the remeasurement of the lease liability as an adjustment to the right-of-use asset.

Lessor accounting under IFRS 16 is substantially unchanged from today’s accounting under IAS 17. Lessors will continue to classify all leases using the same classification principle as in IAS 17 and distinguish between two types of leases: operating and finance leases. IFRS 16 also requires lessees and lessors to make more extensive disclosures than under IAS 17. IFRS 16 is effective for annual periods beginning on or after January 1, 2019. Early application is permitted, but not before an entity applies IFRS 15 Revenue from Contracts with Customers. A lessee can choose to apply the standard using either a full retrospective or a modified retrospective approach. The standard’s transition provisions permit certain reliefs. The Company is currently assessing the potential impact of IFRS 16 on its consolidated financial statements.

Amendments to IFRS 2: Classification and Measurement of Share-based Payment Transactions

The IASB issued amendments to IFRS 2 Share-based Payment that address three main areas: the effects of vesting conditions on the measurement of a cash-settled share-based payment transaction; the classification of a share-based payment transaction with net settlement features for withholding tax obligations; and accounting where a modification to the terms and conditions of a share-based payment transaction changes its classification from cash-settled to equity settled. On adoption, entities are required to apply the amendments without restating prior periods, but retrospective application is permitted if elected for all three amendments and other criteria are met. The amendments are effective for annual periods beginning on or after January 1, 2018, with early application permitted. This amendment is not expected to have a material impact on the Company.

IFRIC Interpretation 22 Foreign Currency Transactions and Advance Consideration

The interpretation clarifies that in determining the spot exchange rate to use on initial recognition of a related asset, expense or income (or part of it) on the derecognition of a non-monetary asset or non-monetary liability relating to advance consideration, the date of the transaction is the date on which an entity initially recognises the non-monetary asset or non-monetary liability arising from the advance consideration. If there are multiple payments or receipts in advance, then the entity must determine a date of the transactions for each payment or receipt of advance consideration. This interpretation is not expected to have a material impact on the Company.

Staff Costs

12 Months Ended

Dec. 31, 2017

Classes Of Employee Benefits Expense [Abstract]

Staff Costs Explanatory

3. Staff costs

Included in research and development expenses:

	2017		2016		2015
	(in thousands)
	£		£		£
Wages and salaries		2,391		2,241		1,366
Social security costs		272		205		151
Pension costs and other benefits		119		102		92
Share-based payments		8,868		1,027		709
		11,650		3,575		2,318

Included in administrative expenses:

	2017		2016		2015
	(in thousands)
	£		£		£
Wages and salaries		408		84		108
Social security costs		37		13		52
Pension costs and other benefits		21		7		15
Share-based payments		2,863		105		76
		3,329		209		251
Total employee benefit expense		14,979		3,784		2,569

The average number of staff employed under contracts of service were:

	2017		2016		2015
	(number)
Research and development activities		18		15		11
Administrative activities		2		1		1
		20		16		12

Directors’ remuneration & other benefits

	2017		2016		2015
	(in thousands)
	£		£		£
Directors’ remuneration		793		549		526
Pension and other benefits		37		34		77
		830		583		603

The number of directors who exercised share options in 2017 is nil (2016: 1; 2015: nil).

The above amounts for remuneration include the following in respect of the highest paid director:

	2017		2016		2015
	(in thousands)
	£		£		£
Directors’ remuneration		601		378		318
Pension and other benefits		33		29		61
		634		407		379

Basic and Diluted Loss Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Basic and diluted loss per share

4. Basic and diluted loss per share

	2017			2016			2015
	(in thousands, except per share data)
	£			£			£
Loss for the year		(23,085	)		(6,049	)		(5,332	)
Basic and diluted weighted average number of shares		26,069			24,107			24,101

	£			£			£
Basic and diluted loss per share		(0.89	)		(0.25	)		(0.22	)

Basic loss per share is calculated by dividing the loss for the year attributable to the equity holders of the Company by the weighted average number of shares outstanding during the year.

The dilutive effect of potential shares through equity settled transactions were considered to be anti-dilutive as they would have decreased the loss per share and were therefore excluded from the calculation of diluted loss per share.

IPO Related Expenses

12 Months Ended

Dec. 31, 2017

Disclosure Of Initial Public Offering Related Expenses [Abstract]

IPO Related Expenses

5. IPO related expenses

	2017		2016		2015
	(in thousands)
	£		£		£
IPO related expenses		1,794		—		—

IPO related costs primarily relate to legal, accounting and other advisors’ fees in relation to the Company’s listing on Nasdaq which was completed on October 2, 2017.

Capital Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Disclosure Of Capital Commitments And Contingencies [Abstract]

Capital commitments and contingencies

6. Capital commitments and contingencies

Other commitments

Collaboration and License Agreements

Cardiff University License

In August 2009, we entered into a research, collaboration and license agreement with Cardiff University and University College Cardiff Consultants Ltd., or Cardiff Consultants, which we refer to as the Cardiff Agreement. The Cardiff Agreement was renewed with an effective date of January 1, 2018 for an additional two years on substantially the same terms. Under the Cardiff Agreement, we collaborate with Cardiff University in the design, synthesis, characterization and evaluation of phosphoramidate prodrugs, which we refer to as ProTides, based on certain nucleosides. We are responsible for funding certain work performed by Cardiff University and making other payments, which we expect will total £292,607 in aggregate in each of 2018 and 2019. Cardiff University and Cardiff Consultants, which is a holder of intellectual property developed by Cardiff University, have assigned to us all rights in the results of the research under the Cardiff Agreement, and agreed not to undertake any research for any competing third party on nucleoside families of interest to us where such research would make use of ProTide-related intellectual property owned or controlled by Cardiff University as of the date of the Cardiff Agreement or which at any time thereafter becomes owned or controlled by Cardiff University, which we refer to as the Cardiff intellectual property, or to grant rights in the Cardiff intellectual property to any third party for use in connection with nucleosides of interest to us. The foregoing restrictions exclude the field of neurodegeneration for one specific nucleoside analog.

6. Capital commitments and contingencies (continued)

Upon our completion of the evaluation of the ProTides, we have the right to select one or more of the evaluated ProTides as candidates for potential development of a commercial product. Cardiff University and Cardiff Consultants have granted us an exclusive worldwide license to use for all purposes the Cardiff intellectual property in respect of the nucleoside family of our selected ProTides. The exclusive dealing obligations of Cardiff University and Cardiff Consultants will continue for these nucleoside families.

On our filing, or that of a sublicensee, of patent applications resulting from research under the Cardiff Agreement, we will owe Cardiff Consultants certain immaterial payments. If we or our sublicensees develop and commercialize a product resulting from such research, we will owe Cardiff Consultants clinical development milestone payments of up to £1,875,000; provided that such milestone payments are due only with respect to the first product within each nucleoside family to achieve the milestone. We will also owe Cardiff Consultants royalties equal to a low-single digit percentage on our sales of a product resulting from such research. Should we sublicense our right to commercialize a product resulting from the research, we will owe Cardiff Consultants a high-single digit percentage of payments received in consideration of the sublicense.

The Cardiff Agreement currently expires on December 31, 2019. Upon expiration, we have the right to extend the period in which we may evaluate products for three months, and for a further three months in exchange for an additional payment. The Cardiff Agreement may also be terminated for an uncured material breach. Licenses to use the Cardiff intellectual property in the development and commercialization of products we have selected for commercialization, and related payment obligations, will survive expiration of the Cardiff Agreement, but not on termination for an uncured material breach.

Cardiff ProTides Agreement

In October 2009, we entered into a license and collaboration agreement with Cardiff ProTides Ltd., or Cardiff ProTides, which agreement was subsequently amended and restated as an assignment, license and collaboration agreement in March 2012 and was further amended in May 2012, which we refer to as the ProTides Agreement. Under the ProTides Agreement, we collaborated with Cardiff ProTides in the discovery, drug design and in vitro screening of purine and pyrimidine based nucleosides as potential drug candidates. We funded certain work at Cardiff ProTides, and Cardiff ProTides has assigned to us all rights in the results of its research under the ProTides Agreement. Cardiff ProTides also assigned to us patents related to certain compounds of interest, including with respect to Acelarin, and granted us an exclusive, worldwide license, including the right to grant sublicenses, to rights in and technical information related to certain unpatented compounds for all therapeutic, diagnostic, prognostic and prophylactic applications.

If we or a sublicensee develop one or more products covered by a valid claim of an assigned patent or patent resulting from Cardiff ProTides’ research, such as Acelarin, we will owe Cardiff ProTides up to approximately $4.5 million in development and approval milestone payments in the aggregate for the first such product. Additional development and approval milestones would be payable for the first additional product in a new nucleoside series covered by a valid claim of an assigned patent or a patent resulting from Cardiff ProTides’ research, although the maximum potential value of such milestone payments is approximately half the value of the milestone payments associated with the first product. We will also owe Cardiff ProTides royalties equal to a percentage in mid- to high-single digits on sales of such products, subject to reduction under certain circumstances. Royalties on sales by sublicensees are set by formula, which formula would be likely to result in a royalty in the mid-single digits.

The ProTides Agreement expires, on a country by country basis, on the later of the expiration, invalidity, abandonment, lapsing or rejection of the last valid claim of an assigned patent or patent resulting from Cardiff ProTides’ research, or, if certain technical information licensed from Cardiff ProTides remains confidential or the product is covered by a period of data exclusivity, ten years from the date of first commercial sale of a product in such country. The ProTides Agreement may be sooner terminated on an uncured material breach, bankruptcy of a party or, by Cardiff ProTides, if we challenge, or assist in a challenge, of the validity or ownership of an assigned patent or patent resulting from Cardiff ProTides’ research, or fail to pay amounts payable under the ProTides Agreement. It may also be sooner terminated where sums payable by us remain unpaid for 45 days after we receive a notice from Cardiff ProTides that the relevant sums are overdue. Upon a termination of the ProTides Agreement, our license rights will terminate except where the breach results from certain breaches by Cardiff ProTides, in which case our license rights continue on a non-exclusive basis, subject to reduced payment obligations. Upon termination of the ProTides Agreement, including as a result of our breach, we will be under an obligation to assign back to Cardiff ProTides the patents which Cardiff ProTides originally assigned to us.

6. Capital commitments and contingencies (continued)

CROs and Manufacturing commitments

We have agreed to make payments to CROs and manufacturers under various CRO and manufacturing agreements. We have not included further details on such contingent payment obligations as the amount, timing and likelihood of such payments are not fixed or determinable.

Commitments under non-cancellable operating leases

Operating leases relate to rental of office space. The Company entered into new lease obligations for office space. The lease obligations are for a period of five years with a break clause after three years. All operating lease contracts contain clauses for market rental reviews on renewal. The Company’s subsidiary renewed its lease agreement for a period of one year. The Company and its subsidiary do not have an option to purchase the leased office at the expiry of the lease periods. Operating lease expense for the years ended December 31, 2017, 2016 and 2015 was £0.2 million, £0.2 million and £0.1 million, respectively.

Future minimum rentals payable under non-cancellable operating leases are as follows:

	2017			2016		2015
	(in thousands)
	£		£			£
Not later than 1 year	158				192		166
Later than 1 year and not later than 5 years	165				62		—
Later than 5 years		—			—		—
	323				254		166

Income Tax Credit

12 Months Ended

Dec. 31, 2017

Major Components Of Tax Expense Income [Abstract]

Income tax credit

7. Income tax credit

The major components of income tax for the years ended December 31, 2017, 2016 and 2015 are as follows:

	2017				2016			2015
	(in thousands)
	£			£				£
Current tax:
In respect of current year U.K.		2,298				1,906			1,182
In respect of current year U.S.		(10	)			(28	)		(6	)
In respect of prior years U.K.		3				235			—
In respect of prior years U.S.		—				3			—
Total current tax		2,291				2,116			1,176

Deferred tax:
In respect of current year U.S.		61				—			—
In respect of prior years U.S.		49				—			—
Total deferred tax		110				—			—

Income tax credit		2,401				2,116			1,176

Current income tax receivable:
U.K. tax		4,207				2,141			1,182
U.S. tax		18				54			—
Current income tax receivable		4,225				2,195			1,182

Deferred tax:
U.S. tax		81				—			—

Current income tax payable:
U.S. tax		—				—			6

The credit for the year can be reconciled to the loss per the statement of operations as follows:

	2017				2016			2015
	(in thousands)
	£			£				£
Loss before tax		(25,486	)			(8,165	)		(6,508	)
Tax on loss at standard U.K. tax rate of 19.25% (2016: 20.00%; 2015: 20.25%)		(4,906	)			(1,633	)		(1,318	)
Effects of:
Expenses not deductible		3,899				1,301			888
Deduction for R&D		(3,051	)			(2,629	)		(1,650	)
Losses surrendered for R&D tax credit		3,051				2,629			1,650
Deferred tax - PY adjustment		(49	)			24			6
Overseas tax payable		10				—			—
R&D tax credit— U.S.		(61	)			—			—
R&D tax credit—current year		(2,298	)			(1,906	)		(1,182	)
R&D tax credit—prior years		(3	)			(235	)		—
Deferred tax asset not recognized		1,007				333			430
Income tax credit		(2,401	)			(2,116	)		(1,176	)

7. Income tax credit (continued)

In the United Kingdom, the Company has not recognized a deferred tax asset in respect of tax losses carried forward as at December 31, 2017 on the basis that the timing during which tax losses could be regarded as recoverable against future taxable profits cannot be determined with reasonable certainty. In the United States, a deferred tax asset has been recognized as management consider that adequate future taxable profits will be available to realize the deferred tax asset.

Temporary differences and cumulative carry forward tax losses for which deferred tax has not been recognized amount to £40.7 million (2016: £32.5 million; 2015: £10.1 million).

U.K. tax legislation, which was substantively enacted on October 26, 2015, includes legislation that will reduce the main rate of U.K. corporation tax from 20% to 18%. This decrease is to be phased in with a reduction to 19%, effective from April 1, 2017, and a reduction to 18%, effective from April 1, 2020. Further, as announced on March 16, 2016, that the full rate of U.K. corporation tax will reduce by a further 1% to 17% from April 1, 2020. This further reduction was included within the U.K. tax legislation, which was substantively enacted on September 6, 2016.

Intangible Assets

12 Months Ended

Dec. 31, 2017

Disclosure Of Intangible Assets [Abstract]

Intangible Assets

8. Intangible assets

	Patents			Computer Software		Total
	(in thousands)
	£		£			£
Cost:
At December 31, 2015		1,004			—		1,004
Additions		529			10		539
At December 31, 2016		1,533			10		1,543
Accumulated amortization:
At December 31, 2015		77			—		77
Charge for the year		86			3		89
At December 31, 2016		163			3		166
Cost:
At December 31, 2016		1,533			10		1,543
Additions		582			143		725
At December 31, 2017		2,115			153		2,268
Accumulated amortization:
At December 31, 2016		163			3		166
Charge for the year		139			25		164
At December 31, 2017		302			28		330
Net book value:
At December 31, 2017		1,813			125		1,938
At December 31, 2016		1,370			7		1,377

Property, Plant and Equipment

12 Months Ended

Dec. 31, 2017

Disclosure Of Property Plant And Equipment [Abstract]

Property, Plant and Equipment

9. Property, plant and equipment

	Office and computer equipment			Fixtures and Fittings		Total
	(in thousands)
	£			£		£
Cost:
At December 31, 2015		37			—		37
Additions		15			—		15
Disposals		(1	)		—		(1	)
Effect of foreign currency exchange differences		1			—		1
At December 31, 2016		52			—		52
Depreciation:
At December 31, 2015		23			—		23
Charge for the year		12			—		12
Disposals		(1	)		—		(1	)
Effect of foreign currency exchange differences		—			—		—
At December 31, 2016		34			—		34
Cost:
At December 31, 2016		52			—		52
Additions		90			280		370
Disposals		—			—		—
Effect of foreign currency exchange differences		—			—		—
At December 31, 2017		142			280		422
Depreciation:
At December 31, 2016		34			—		34
Charge for the year		17			13		30
Disposals		—			—		—
Effect of foreign currency exchange differences		—			—		—
At December 31, 2017		51			13		64
Net book value:
At December 31, 2017		91			267		358
At December 31, 2016		18			—		18

Prepayments, Accrued Income and Other Receivables

12 Months Ended

Dec. 31, 2017

Prepayments Accrued Income And Other Receivables [Abstract]

Prepayments, accrued income and other receivables

10. Prepayments, accrued income and other receivables

	2017			2016
	(in thousands)
	£		£
Prepayments—manufacturing and clinical		1,979			3,050
Prepayments—other		522			173
Accrued income		67			21
VAT		473			198
Other receivables		9			192
		3,050			3,634

Share Capital and Share Premium

12 Months Ended

Dec. 31, 2017

Disclosure Of Classes Of Share Capital [Abstract]

Share Capital and Share Premium

11. Share capital and share premium

	2017		2016		2015
	(in thousands)
	£		£		£
Share capital		1,272		663		659
Share premium		79,236		42,770		42,574
		80,508		43,433		43,233

	Number		Number		Number
	(in thousands)
Issued share capital comprises:
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)		31,811		6,239		6,156
Founder ordinary 1 shares of £0.01 each		—		1,000		1,000
Founder ordinary 2 shares of £0.01 each		—		1,000		1,000
Series A shares of £0.01 each		—		7,483		7,483
Series B shares of £0.001 each		—		8,463		8,463
		31,811		24,185		24,102

The table above does not total due to rounding.

In order to facilitate the Company being re-registered as a public company, the directors of the Company signed a solvency statement on June 29, 2017 with the agreement of the shareholders and undertook a capital reduction reducing its share premium by £42.5 million, which was credited to the Company’s capital reserve.

On September 14, 2017, the Company completed a one-for-four reverse share split and an associated bonus allotment of shares to take into account fractional entitlements. This had the effect of consolidating every four ordinary shares of £0.01 to one ordinary share of £0.04, every four founder ordinary 1 shares of £0.01 to one founder ordinary 1 share of £0.04, every four founder ordinary 2 shares of £0.01 to one founder ordinary 2 share of £0.04, every four series A shares of £0.01 to one series A share of £0.04 and every four series B shares of £0.001 to one series B share of £0.004.

Following the one-for-four reverse share split, for the purpose of facilitating a conversion of each Series B share (nominal value £0.004 per share), into an ordinary share (nominal value £0.04 per share), the company allotted to holders of Series B shares an additional nine Series B shares for each Series B share held. Subjected to and conditional upon this allotment, every 10 Series B shares of £0.004 were consolidated into a single Series B share of £0.04. Each Series B share of £0.04 was then automatically converted into one ordinary share of £0.04. The company funded the allotment of these additional shares with reserves that were standing to the credit of the share premium account. The impact of this bonus issue was £304,650.

Immediately prior to the initial public offering, all issued series A convertible participating shares, series B convertible participating shares, founder ordinary 1 shares and founder ordinary 2 shares were converted into ordinary shares on a one-for-one basis. The Company had 24,214,641 shares outstanding. This included an issue of 30,000 shares in August 2017, upon the exercise of options. On October 2, 2017, the Company completed an IPO of 7,596,505 American Depository Shares (ADSs) at a price to the public of $15.00 per ADS. Each ADS represents one ordinary share of the Company.

11. Share capital and share premium (continued)

	Number of shares		Share capital		Share premium
	(in thousands)
			£		£
Fully paid shares:
Balance at January 1, 2015 and December 31, 2015		24,102		659		42,574
Issue of shares on exercise of options		83		4		196
Balance at December 31, 2016		24,185		663		42,770
Reduction in share premium account		—		—		(42,466	)
Exercise of share options		30		1		119
Bonus issue to series B		—		304		(304	)
Issue of share capital		7,596		304		79,530
IPO costs		—		—		(413	)
Balance at December 31, 2017		31,811		1,272		79,236

Ordinary shares

Prior to the re-organization of capital described above, the ordinary shares ranked equally with all other shares in issue in that on a poll every member had one vote for each ordinary share held (save for the enhanced voting rights referred to in the founder ordinary shares). The ordinary shares ranked equally with all other shares in issue in respect of any rights to any dividend distribution. The ordinary shares ranked equally with all other shares in issue in respect of any rights to any capital distribution. The ordinary shares were not redeemable.

After the re-organization of capital described above, holders of ordinary shares are entitled to one vote for each share held of record on all matters submitted to a vote of shareholders and do not have cumulative voting rights.

Founder ordinary shares

Upon shareholder vote, the founder ordinary 1 shares and the founder ordinary 2 shares as separate classes of shares each conferred upon the holders of such classes of shares such number of votes, which equaled at least 5% of all votes exercisable by all holders of shares.

Series A shares

The series A shares ranked equally with all other shares in issue in that on a vote every member had one vote for each series A share held (save for the enhanced voting rights conferred upon the founder ordinary shares). The series A shares ranked equally with all others shares in respect of any rights to any dividend distribution. The series A shares ranked equally with all other shares in issue in respect of any rights to any capital distribution. The series A shares were not redeemable.

Series B shares

The series B shares ranked equally with all other shares in issue (save for the enhanced voting rights conferred upon the founder ordinary shares). The series B shares ranked equally with all other shares in issue in respect of any rights to any dividend distribution. The series B shares ranked equally with all other shares in respect of any rights to any capital distribution. The series B shares were not redeemable.

Capital management

For the purpose of the Company’s capital management, capital includes issued capital, share premium and all other equity reserves attributable to the equity holders of the Company. The purpose of the Company’s capital management is to maximize shareholder value and ensure adequate capital is available to meet the medium-term operating plan. Review of operations and commitments is key to identifying future capital management and a full review is undertaken on a quarterly basis.

No changes were made in the objectives, policies or processes for managing capital during the year ended December 31, 2017, 2016 or 2015.

Other Reserves

12 Months Ended

Dec. 31, 2017

Disclosure Of Other Reserves [Abstract]

Other reserves

12. Other reserves

	2017			2016			2015
	(in thousands)
	£			£			£
Own share reserve		(339	)		(339	)		(339	)
Foreign currency translation reserve		(11	)		(3	)		(1	)
Capital reserve		42,466			—			—
Share option reserve
Balance at beginning of year		4,406			3,291			2,506
Share-based payments		11,731			1,132			785
Exercise of share options		(180	)		(17	)		—
Lapse of share options		(2	)		—			—
Balance at end of year		15,955			4,406			3,291
Total other reserves		58,071			4,064			2,951

Foreign currency translation reserve

The foreign currency translation reserve is used to record exchange differences arising from the translation of the financial statements of foreign operations.

Own share reserve

The own share reserve represents the cost of 500,000 shares of NuCana plc purchased by NuCana Employee Benefit Trust and that may, at the discretion of the trustee, be used to satisfy future exercise of options under the Company’s share options plan.

Capital reserve

The capital reserve balance arose from the reduction of our share premium account and corresponding increase to our capital reserve account reflected as of June 30, 2017 in connection with our re-registration as a public limited company, as further described in Note 1.

Share option reserve

The share option reserve is used to recognize the value of equity-settled share-based payments provided to employees, directors and consultants as part of their remuneration. Refer to Note 13 for further details of these plans.

Share-based payments

12 Months Ended

Dec. 31, 2017

Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]

Share-based payments

13. Share-based payments

The Company has three share-based payment plans for employees, directors and consultants. The share options granted will be settled in equity.

2016 options

In 2016, share options were granted under the following share-based payment plans:

U.K. share-based payment plans

Options granted under these plans will vest if the option holder remains under their respective employment/consultancy contract for the agreed vesting period. The majority of the share options granted under these plans will vest equally over a period of four years, with the exception of the following:

1) options granted to a director, under which a third of the options granted vested immediately with the remaining two-thirds vesting each subsequent year; and

13. Share-based payments (continued)

2) options granted to three employees, under which two thirds of the options vested immediately with the remaining third vesting in 2017.

Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date.

Stock option plan (U.S. Sub-Plan)

On June 30, 2016, share options were granted under this sub-plan, which will vest equally over a period of four years if the option holder remains under the respective employment contract. Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date.

On December 12, 2016, 45,750 share options were granted to a non-executive director with an exercise price of £4.00 each, which could be exercised within six months after the grant date. The share options were exercised on December 29, 2016. In terms of the stock option agreement, should that non-executive director cease holding office within the four years from the grant date then a reducing number, dependent upon when such cessation occurs, of these shares options so acquired may, at the Board’s discretion, be subject to a mandatory transfer notice.

These share options will only be fully unencumbered by such restriction after a period of four years from the expiration of the grant date and consequently the share-based payment expense is recognized on a straight-line basis.

Cancelled share option arrangement

During the year ended December 31, 2016, the share-based payment arrangements for three employees were cancelled. The cancellation of the original plan resulted in an acceleration of the remaining vesting period, with the remaining charge recognized in 2016 deemed immaterial.

2017 options

In 2017, share options were granted under the following share-based payment plans:

U.K. share-based payment plans

Options granted under these plans will vest if the option holder remains under their respective contract of employment or contract of service for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years, with the exception of options granted to a director, under which the options granted vested immediately.

Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date.

Stock option plan (U.S. Sub-Plan)

Options granted under these plans will vest if the option holder remains under their respective employment contract for the agreed vesting period. The share options granted under these plans will vest equally over a period of four years.

13. Share-based payments (continued)

Upon vesting, each option allows the holder to purchase one ordinary share at a specified option price determined at grant date.

Share options and weighted average exercise prices are as follows for the reporting periods presented:

	Number of shares			Weighted average exercise price per share
				£
Outstanding at January 1, 2015		2,873,747			0.26
Granted		—			—
Forfeited		—			—
Exercised		—			—
Outstanding at December 31, 2015		2,873,747			0.26
Granted		507,690			3.81
Forfeited		—			—
Cancelled		(50,000	)		2.95
Exercised¹		(83,250	)		2.40
Outstanding at December 31, 2016		3,248,187			0.72
Granted		1,500,815			2.58
Forfeited		(7,500	)		0.16
Cancelled		—			—
Exercised²		(30,000	)		4.00
Outstanding at December 31, 2017		4,711,502			1.29
Vested and exercisable at December 31, 2017³		4,030,833			0.36
Vested and exercisable at December 31, 2016		2,841,419			0.36
Vested and exercisable at December 31, 2015		2,262,500			0.23

1. The weighted average share price at the date of exercise of these options was £9.00

2. The weighted average share price at the date of exercise of these options was £10.43

3. Share options granted to a non-executive director in 2016 are not included in these calculations. These share options were exercised and will only be fully unencumbered after a period of four years from the grant date.

The weighted average remaining contractual life of the share options outstanding as at December 31, 2017 is 4.49 years (2016: 6.66; 2015: 6.18).

13. Share-based payments (continued)

The following principal assumptions were used in the valuation for the 2016 share options.

		Options granted on
		Jun 30, 2016				Aug 22, 2016				Aug 22, 2016				Aug 22, 2016				Aug 22, 2016
Vesting dates		Oct 12, 2016				Aug 22, 2016				Aug 22, 2016				Dec 07, 2016				Mar 01, 2017
		Oct 12, 2017				Aug 22, 2017				Aug 22, 2017				Dec 07, 2017				Mar 01, 2018
		Oct 12, 2018												Dec 07, 2018				Mar 01, 2019
		Oct 12, 2019												Dec 07, 2019				Mar 01, 2020
Volatility			69.05	%			67.92	%			67.92	%			67.92	%			67.92	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.11	%			0.07	%			0.07	%			0.07	%			0.07	%
Fair value of option at grant date	£		4.02		£		6.15		£		6.01		£		5.74		£		5.62
Fair value of share at grant date	£		6.97		£		8.61		£		8.61		£		8.61		£		8.61
Exercise price at date of grant	£		4.00		£		2.80		£		3.00		£		3.40		£		3.60
Lapse date		June 30, 2026				Aug 22, 2026				Aug 22, 2026				Aug 22, 2026				Aug 22, 2026
Expected option life (years)			2.50				2.36				2.36				2.36				2.36
Number of options granted			270,690				12,500				37,500				25,000				11,250
		Options granted on
		Aug 22, 2016				Aug 22, 2016				Oct 28, 2016				Oct 28, 2016				Dec 12, 2016
Vesting dates		May 01, 2017				May 16, 2017				Oct 28, 2017				Oct 28, 2016				Dec 12, 2017
		May 01, 2018				May 16, 2018				Oct 28, 2018				Oct 28, 2017				Dec 12, 2018
		May 01, 2019				May 16, 2019				Oct 28, 2019				Oct 28, 2018				Dec 12, 2019
		May 01, 2020				May 16, 2020				Oct 28, 2020								Dec 12, 2020
Volatility			67.92	%			67.92	%			68.38	%			68.38	%			60.92	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.07	%			0.07	%			0.28	%			0.28	%			0.06	%

Fair value of option at grant date	£		5.62		£		5.62		£		5.85		£		5.46		£		5.22
Fair value of share at grant date	£		8.61		£		8.61		£		8.76		£		8.76		£		9.19
Exercise price at date of grant	£		3.60		£		3.60		£		3.40		£		4.00		£		4.00
Lapse date		Aug 22, 2026				Aug 22, 2026				Oct 28, 2026				Oct 28, 2026				Dec 12, 2026
Expected option life (years)			2.36				2.36				2.17				2.17				0.50
Number of options granted			5,000				37,500				12,500				50,000				45,750

The fair values of options granted were determined using the Black-Scholes model that takes into account factors specific to the share incentive plan such as the assumption that the options will be exercised at a single point in time, in December 2018. This has been incorporated into the measurement by means of actuarial modelling. As NuCana plc was unlisted at the grant date of the options, it is not possible to derive historical volatility from the Company’s own share price. The underlying expected volatility was therefore determined by using the historical volatility of similar listed entities as a proxy. The volatility percentage applied to each tranche is the average of the historical volatility of companies comparable to NuCana plc. In prior years, management made contemporaneous valuation of the share price, based on recent capital transactions, as an input to the Black-Scholes model. For the 2016 awards, the Company’s ordinary share valuations were prepared using the guideline public company, or GPC, method under the market approach. In the application of the GPC method, we considered the pricing of IPOs completed by clinical-stage oncology companies between April 2015 and May 2016. We converted prospective IPO value to present value by applying a discount rate of 25%. The discount rate was derived from studies of rates of return required by venture investors in IPO-stage companies. In addition to the IPO GPCs, we considered the enterprise values indicated by a group of eight trading GPCs. The trading prices of these clinical-stage GPCs provided contemporaneous indications of value as of each appraisal date. We applied a discount for lack of marketability to the ordinary shares to account for the lack of access to an active public market. We estimated the discount for lack of marketability using an Asian put model. In the year ended December 31, 2016, an employee remuneration expense, all of which related to equity-settled share-based payments, of £1.1 million (2015: £0.8 million) has been included in the statement of operations and credited to equity.

13. Share-based payments (continued)

The following principal assumptions were used in the valuation for share 2017 options.

		May 16, 2017				Sep 13, 2017				Sep 14, 2017				Sep 14, 2017				Sep 14, 2017
Vesting dates		Oct 28, 2017				Sep 13, 2018				Mar 06, 2018				Sep 14, 2018				Sep 14, 2018
		Oct 28, 2018				Sep 13, 2019				Mar 06, 2019				Sep 14, 2019				Sep 14, 2019
		Oct 28, 2019				Sep 13, 2020				Mar 06, 2020				Sep 14, 2020				Sep 14, 2020
		Oct 28, 2020				Sep 13, 2021				Mar 06, 2012				Sep 14, 2021				Sep 14, 2021
Volatility			66.56	%			66.94	%			66.97	%			66.97	%			66.97	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.12	%			0.27	%			0.36	%			0.36	%			0.36	%
Fair value of option at grant date	£		7.70		£		6.15		£		6.08		£		6.08		£		6.08
Fair value of share at grant date	£		11.08		£		10.43		£		10.34		£		10.34		£		10.34
Exercise price at date of grant	£		4.00		£		5.40		£		5.40		£		5.40		£		5.40
Lapse date		May 16, 2027				Sep 13, 2027				Sep 14, 2027				Sep 14, 2027				Sep 14, 2027
Expected option life (years)			2.63				2.30				2.30				2.30				2.30
Number of options granted			23,250				14,690				25,000				110,310				12,500

		Sep 14, 2017				Sep 15, 2017				Sep 15, 2017				Sep 27, 2017				Sep 27, 2017
Vesting dates		Sep 14, 2018				Sep 15, 2018				Sep 15, 2017				Mar 20, 2018				Sep 27, 2018
		Sep 14, 2019				Sep 15, 2019								Mar 20, 2019				Sep 27, 2019
		Sep 14, 2020				Sep 15, 2020								Mar 20, 2020				Sep 27, 2020
		Sep 14, 2021				Sep 15, 2021								Mar 20, 2021				Sep 27, 2021
Volatility			66.97	%			67.02	%			67.02	%			67.11	%			67.11	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.36	%			0.44	%			0.44	%			0.47	%			0.47	%
Fair value of option at grant date	£		3.90		£		6.76		£		10.11		£		4.36		£		4.36
Fair value of share at grant date	£		10.34		£		10.15		£		10.15		£		11.19		£		11.19
Exercise price at date of grant	£		10.80		£		4.00		£		0.04		£		11.19		£		11.19
Lapse date		Sep 14, 2027				Sep 15, 2027				Sep 15, 2027				Sep 27, 2027				Sep 27, 2027
Expected option life (years)			2.30				2.29				2.29				2.26				2.26
Number of options granted			25,000				45,750				1,028,533				37,500				178,282

The fair values of options granted were determined using the Black-Scholes model that takes into account factors specific to the share incentive plan such as the assumption that the options will be exercised at a single point in time, in December 2019. This has been incorporated into the measurement by means of actuarial modelling. As NuCana plc was unlisted at the grant date of the options, it is not possible to derive historical volatility from the Company’s own share price. The underlying expected volatility was therefore determined by using the historical volatility of similar listed entities as a proxy. The volatility percentage applied to each tranche is the average of the historical volatility of companies comparable to NuCana plc. With the exception of the awards granted on September 27, 2017, the Company’s ordinary share valuations were prepared using the guideline public company, or GPC, method under the market approach. In the application of the GPC method, we considered the pricing of IPOs completed by clinical-stage oncology companies between July 2015 and June 2017. We converted prospective IPO value to present value by applying a discount rate of 25%. The discount rate was derived from studies of rates of return required by venture investors in IPO-stage companies. In addition to the IPO GPCs, we considered the enterprise values indicated by a group of eight trading GPCs. The trading prices of these clinical-stage GPCs provided contemporaneous indications of value as of each appraisal date. We applied a discount for lack of marketability to the ordinary shares to account for the lack of access to an active public market. We estimated the discount for lack of marketability using an Asian put model. In the year ended December 31, 2017, an employee remuneration expense, all of which related to equity-settled share-based payments, of £11.7 million (2016: £1.1 million; 2015: £0.8 million) has been included in the statement of operations and credited to equity.

Related party disclosures

12 Months Ended

Dec. 31, 2017

Related party disclosures

14. Related party disclosures

Compensation of key management personnel of the Company

	2017		2016		2015
	(in thousands)
	£		£		£
Short-term employee benefits		1,184		1,053		647
Severance payments		—		150		—
Pension and other benefits		56		47		77
Share-based payments		11,230		731		726
		12,470		1,981		1,450

The amounts disclosed in the table above are the amounts recognized as an expense during the reporting year related to key management personnel.

Cash, Cash Equivalents And Short-Term Deposits

12 Months Ended

Dec. 31, 2017

Cash And Cash Equivalents [Abstract]

Cash, cash equivalents and short-term deposits

15. Cash, cash equivalents and short-term deposits

	2017		2016
	(in thousands)
	£		£
Cash and cash equivalents		86,703		19,990

Cash and cash equivalents comprise cash at banks with maturity of three months or less, which is subject to insignificant risk of changes in value. Cash at banks earn interest at fixed or variable rates based on the terms agreed for each account.

Liquidity risk is minimal and is managed using deposits with immediate and varied fixed term dates.

Cash, cash equivalents and short term deposits are measured at fair value and classified as Level 1 under the fair value level hierarchy.

Financial Instruments

12 Months Ended

Dec. 31, 2017

Disclosure Of Financial Instruments [Abstract]

Financial Instruments

16. Financial instruments

Fair value of financial assets and liabilities

The fair value of the financial assets and liabilities is included at the amount at which an instrument could be exchanged in a current transaction between willing parties, other than in a forced liquidation or sale.

The following methods and assumptions were used to estimate the fair values:

Cash, cash equivalents, short-term deposits, other receivables, trade payables and other payables approximate their carrying amounts largely due to the short-term maturities of these instruments.

Market Risk

Market risk arises from our exposure to fluctuation in interest rates and currency exchange rates. These risks are managed by maintaining an appropriate mix of cash deposits in the two main currencies we operate in, placed with a variety of financial institutions for varying periods according to expected liquidity requirements.

16. Financial instruments (continued)

Interest Rate Risk

As of December 31, 2017, we had cash and cash equivalents of £86.7 million. As of December 31, 2016, we had cash and cash equivalents of £20.0 million. Our exposure to interest rate sensitivity is impacted primarily by changes in the underlying U.K. bank interest rates. Our surplus cash and cash equivalents are invested in interest-bearing accounts and certificates of deposit from time to time which earn interest at fixed or variable rates based on the terms agreed for each account. We have not entered into investments for trading or speculative purposes.

Financial assets subject to fixed or variable interest rates are as follows:

	2017		2016
	(in thousands)
	Carrying amount		Carrying amount
	£		£
Financial assets at fixed rates
Cash and cash equivalents		51,745		5,000
Financial assets at variable rates
Cash and cash equivalents		13,708		14,433

An increase in the U.K. bank interest rates by 0.5 percentage points would increase the net annual interest income applicable to the cash and cash equivalents by £327,261 (2016: £97,165).

Currency Risk

Our functional currency is the U.K. pound sterling, and our transactions are commonly denominated in that currency. However, we incur a portion of expenses in other currencies, primarily U.S. dollars, and are exposed to the effects of this exchange rate. Since mid-2016, there has been significantly increased volatility in the exchange rate between the pound sterling and the U.S. dollar and an overall weakening of the pound sterling related to Britain’s exit from the European Union. Although we are based in the United Kingdom, we source our active pharmaceutical ingredient, or API, and other raw materials and our research and development, manufacturing, consulting and other services worldwide, including from the United States, the European Union and India.

Any weakening of the pound sterling against the currencies of such other jurisdictions makes the purchase of such goods and services more expensive for us. We seek to minimize this exposure by maintaining currency cash balances at levels appropriate to meet foreseeable short to mid-term expenses in these other currencies. We do not use forward exchange contracts to manage exchange rate exposure.

Financial assets and liabilities in foreign currencies are as follows:

	2017		2016
	(in thousands)
	Carrying amount		Carrying amount
	£		£
Financial assets
Prepayments, accrued income and other receivables		2,656		3,068
Current income tax receivable		18		54
Cash and cash equivalents		72,645		251

Financial liabilities
Trade payables		148		126
Payroll taxes and social security		3		—
Accrued expenditure		468		167

16. Financial instruments (continued)

A 1% increase in exchange rates would reduce the carrying value of net financial assets and liabilities in foreign currencies by £747,004 (2016: £30,807).

Credit risk

We actively manage our cash and cash equivalents across a number of banks and have deposits with different maturity dates. The Company monitors the credit rating of those banks.

Events After the Reporting Period	12 Months Ended
Events After the Reporting Period	Dec. 31, 2017
Events After Reporting Period [Abstract]
Events After the Reporting Period	17. Events after the reporting period There have been no significant changes to the Company’s circumstances since the year-end.

Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Disclosure Of Summary Of Significant Accounting Policies [Abstract]

Basis of preparation

The financial statements have been prepared in accordance with International Financial Reporting Standards (“IFRS”) as issued by the International Accounting Standards Board (“IASB”).

Going concern

In common with many companies in the biopharmaceutical sector, the Company incurs significant expenditure in its early years as it researches and develops its potential products for market.

Judgements and estimates

The following judgements have had the most significant effect on the amounts recognized in the financial statements:

Research and development expenses

Taxation

The following estimates have had the most significant effect on the amounts recognized in the financial statements:

Recognition of Clinical Trial Expenses

•

fees paid to Clinical Research Organizations (CROs) for services performed on preclinical studies and clinical trials; and

•

fees paid for professional services.

Share-based payments

Basis of consolidation

The consolidated financial statements comprise the financial statements of the Company and its subsidiaries.

2. Significant accounting policies (continued)

Segment reporting

Initial public offering (IPO) related expenses

Property, plant and equipment

Depreciation is provided on property, plant and equipment over their expected useful economic life as follows:

Asset class

Depreciation method and period

Office and computer equipment

Fixtures and fittings

Straight-line over 3 years

Straight-line over 5 years

Intangible assets

The amortization method and amortization period for the principal categories of intangible assets are follows:


Asset class		Amortization method and period
Patents		Straight-line over 20 years
Computer software		Straight-line between 3 and 5 years

Intangible assets are tested for impairment when there is an indicator of impairment.

Cash, cash equivalents and short-term deposits

2. Significant accounting policies (continued)

Cash, cash equivalents and short-term deposits

Cash and cash equivalents in the statement of financial position include cash at banks with a maturity of less than three months, which is subject to an insignificant risk of changes in value.

Short-term deposits represent certificates of deposits with banks with maturities of greater than three months but less than one year.

Research and development

•

the technical feasibility of completing the intangible asset so that the asset will be available for use or sale;

•

its intention to complete and its ability and intention to use or sell the asset;

•

how the asset will generate future economic benefits;

•

the availability of resources to complete the asset; and

•

the ability to measure reliably the expenditure during development.

Income Taxes

Current income tax

Deferred income tax

Income tax credit

Operating leases

Impairment of non-financial assets

The Company assesses, at each reporting date, whether there is an indication that an asset may be impaired. If any indication exists, the Company estimates the asset’s recoverable amount.

A cash-generating unit is the smallest identifiable group of assets that generates cash inflows that are largely independent of the cash inflows from other assets or groups of assets.

Calculation of recoverable amount

Reversal of impairment

Share-based payments

Fair value measurement

Level 1: Quoted (unadjusted) prices in active markets for identical assets or liabilities.

Level 2: Other techniques for which all inputs that have a significant effect on the recorded fair value are observable, either directly or indirectly.

Level 3: Techniques that use inputs that have a significant effect on the recorded fair value that are not based on observable market data.

Accounting Standards

Amendments to IAS 7 Statement of Cash Flows: Disclosure Initiative

Amendments to IAS 12 Income Taxes: Recognition of Deferred Tax Assets for Unrealised Losses

Annual Improvements 2014-2016 Cycle

These improvements include:

IFRS 12 Disclosure of Interests in Other Entities: Clarification of the scope of disclosure requirements in IFRS 12

The IASB and IFRIC have issued the following standards and interpretations, which are considered relevant to the Company, with an effective date after the date of these financial statements.

IFRS 9: Financial Instruments

IFRS 16: Leases

Amendments to IFRS 2: Classification and Measurement of Share-based Payment Transactions

IFRIC Interpretation 22 Foreign Currency Transactions and Advance Consideration

Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Summary Of Significant Accounting Policies [Abstract]

Summary of Depreciation on Property, Plant and Equipment Over Their Expected Useful Economic Life

Depreciation is provided on property, plant and equipment over their expected useful economic life as follows:

Asset class

Depreciation method and period

Office and computer equipment

Fixtures and fittings

Straight-line over 3 years

Straight-line over 5 years

Summary of Amortization Method and Amortization Period for the Principal Categories of Intangible Assets

The amortization method and amortization period for the principal categories of intangible assets are follows:


Asset class		Amortization method and period
Patents		Straight-line over 20 years
Computer software		Straight-line between 3 and 5 years

Staff Costs (Tables)

12 Months Ended

Dec. 31, 2017

Staff Costs [Line Items]

Schedule of Average Number of Staff Employed Under Contracts of Service Explanatory

The average number of staff employed under contracts of service were:

	2017		2016		2015
	(number)
Research and development activities		18		15		11
Administrative activities		2		1		1
		20		16		12

Summary of Directors’ Remuneration and Other Benefits

Directors’ remuneration & other benefits

	2017		2016		2015
	(in thousands)
	£		£		£
Directors’ remuneration		793		549		526
Pension and other benefits		37		34		77
		830		583		603

Highest Paid Director

Staff Costs [Line Items]

Summary of Directors’ Remuneration and Other Benefits

The above amounts for remuneration include the following in respect of the highest paid director:

	2017		2016		2015
	(in thousands)
	£		£		£
Directors’ remuneration		601		378		318
Pension and other benefits		33		29		61
		634		407		379

Research and Development Expenses

Staff Costs [Line Items]

Disclosure of Employee Benefit Expense

Included in research and development expenses:

	2017		2016		2015
	(in thousands)
	£		£		£
Wages and salaries		2,391		2,241		1,366
Social security costs		272		205		151
Pension costs and other benefits		119		102		92
Share-based payments		8,868		1,027		709
		11,650		3,575		2,318

Administrative Expenses

Staff Costs [Line Items]

Disclosure of Employee Benefit Expense

Included in administrative expenses:

	2017		2016		2015
	(in thousands)
	£		£		£
Wages and salaries		408		84		108
Social security costs		37		13		52
Pension costs and other benefits		21		7		15
Share-based payments		2,863		105		76
		3,329		209		251
Total employee benefit expense		14,979		3,784		2,569

Basic and Diluted Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Schedule of Basic and Diluted Loss Per Share

	2017			2016			2015
	(in thousands, except per share data)
	£			£			£
Loss for the year		(23,085	)		(6,049	)		(5,332	)
Basic and diluted weighted average number of shares		26,069			24,107			24,101

	£			£			£
Basic and diluted loss per share		(0.89	)		(0.25	)		(0.22	)

IPO Related Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Initial Public Offering Related Expenses [Abstract]

Summary of Initial Public Offering Related Expenses

	2017		2016		2015
	(in thousands)
	£		£		£
IPO related expenses		1,794		—		—

Capital Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Capital Commitments And Contingencies [Abstract]

Summary of Capital Commitments

Future Minimum Rentals Payable Under Non-cancellable Operating Leases

Future minimum rentals payable under non-cancellable operating leases are as follows:

	2017			2016		2015
	(in thousands)
	£		£			£
Not later than 1 year	158				192		166
Later than 1 year and not later than 5 years	165				62		—
Later than 5 years		—			—		—
	323				254		166

Income Tax Credit (Tables)

12 Months Ended

Dec. 31, 2017

Major Components Of Tax Expense Income [Abstract]

Schedule of Major Components of Income Tax

The major components of income tax for the years ended December 31, 2017, 2016 and 2015 are as follows:

	2017				2016			2015
	(in thousands)
	£			£				£
Current tax:
In respect of current year U.K.		2,298				1,906			1,182
In respect of current year U.S.		(10	)			(28	)		(6	)
In respect of prior years U.K.		3				235			—
In respect of prior years U.S.		—				3			—
Total current tax		2,291				2,116			1,176

Deferred tax:
In respect of current year U.S.		61				—			—
In respect of prior years U.S.		49				—			—
Total deferred tax		110				—			—

Income tax credit		2,401				2,116			1,176

Current income tax receivable:
U.K. tax		4,207				2,141			1,182
U.S. tax		18				54			—
Current income tax receivable		4,225				2,195			1,182

Deferred tax:
U.S. tax		81				—			—

Current income tax payable:
U.S. tax		—				—			6

Schedule of Reconciliation of Accounting Loss per Statement of Operations

The credit for the year can be reconciled to the loss per the statement of operations as follows:

	2017				2016			2015
	(in thousands)
	£			£				£
Loss before tax		(25,486	)			(8,165	)		(6,508	)
Tax on loss at standard U.K. tax rate of 19.25% (2016: 20.00%; 2015: 20.25%)		(4,906	)			(1,633	)		(1,318	)
Effects of:
Expenses not deductible		3,899				1,301			888
Deduction for R&D		(3,051	)			(2,629	)		(1,650	)
Losses surrendered for R&D tax credit		3,051				2,629			1,650
Deferred tax - PY adjustment		(49	)			24			6
Overseas tax payable		10				—			—
R&D tax credit— U.S.		(61	)			—			—
R&D tax credit—current year		(2,298	)			(1,906	)		(1,182	)
R&D tax credit—prior years		(3	)			(235	)		—
Deferred tax asset not recognized		1,007				333			430
Income tax credit		(2,401	)			(2,116	)		(1,176	)

Intangible Assets (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Intangible Assets [Abstract]

Summary of Intangible Assets

	Patents			Computer Software		Total
	(in thousands)
	£		£			£
Cost:
At December 31, 2015		1,004			—		1,004
Additions		529			10		539
At December 31, 2016		1,533			10		1,543
Accumulated amortization:
At December 31, 2015		77			—		77
Charge for the year		86			3		89
At December 31, 2016		163			3		166
Cost:
At December 31, 2016		1,533			10		1,543
Additions		582			143		725
At December 31, 2017		2,115			153		2,268
Accumulated amortization:
At December 31, 2016		163			3		166
Charge for the year		139			25		164
At December 31, 2017		302			28		330
Net book value:
At December 31, 2017		1,813			125		1,938
At December 31, 2016		1,370			7		1,377

Property, Plant and Equipment (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Property Plant And Equipment [Abstract]

Summary of Property, Plant and Equipment

	Office and computer equipment			Fixtures and Fittings		Total
	(in thousands)
	£			£		£
Cost:
At December 31, 2015		37			—		37
Additions		15			—		15
Disposals		(1	)		—		(1	)
Effect of foreign currency exchange differences		1			—		1
At December 31, 2016		52			—		52
Depreciation:
At December 31, 2015		23			—		23
Charge for the year		12			—		12
Disposals		(1	)		—		(1	)
Effect of foreign currency exchange differences		—			—		—
At December 31, 2016		34			—		34
Cost:
At December 31, 2016		52			—		52
Additions		90			280		370
Disposals		—			—		—
Effect of foreign currency exchange differences		—			—		—
At December 31, 2017		142			280		422
Depreciation:
At December 31, 2016		34			—		34
Charge for the year		17			13		30
Disposals		—			—		—
Effect of foreign currency exchange differences		—			—		—
At December 31, 2017		51			13		64
Net book value:
At December 31, 2017		91			267		358
At December 31, 2016		18			—		18

Prepayments, Accrued Income and Other Receivables (Tables)

12 Months Ended

Dec. 31, 2017

Prepayments Accrued Income And Other Receivables [Abstract]

Summary of Prepayments, Accrued Income and Other Receivables

	2017			2016
	(in thousands)
	£		£
Prepayments—manufacturing and clinical		1,979			3,050
Prepayments—other		522			173
Accrued income		67			21
VAT		473			198
Other receivables		9			192
		3,050			3,634

Share Capital and Share Premium (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Classes Of Share Capital [Abstract]

Summary of Share Capital and Share Premium

	2017		2016		2015
	(in thousands)
	£		£		£
Share capital		1,272		663		659
Share premium		79,236		42,770		42,574
		80,508		43,433		43,233

	Number		Number		Number
	(in thousands)
Issued share capital comprises:
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)		31,811		6,239		6,156
Founder ordinary 1 shares of £0.01 each		—		1,000		1,000
Founder ordinary 2 shares of £0.01 each		—		1,000		1,000
Series A shares of £0.01 each		—		7,483		7,483
Series B shares of £0.001 each		—		8,463		8,463
		31,811		24,185		24,102

Summary of Movement in the Share Capital

11. Share capital and share premium (continued)

	Number of shares		Share capital		Share premium
	(in thousands)
			£		£
Fully paid shares:
Balance at January 1, 2015 and December 31, 2015		24,102		659		42,574
Issue of shares on exercise of options		83		4		196
Balance at December 31, 2016		24,185		663		42,770
Reduction in share premium account		—		—		(42,466	)
Exercise of share options		30		1		119
Bonus issue to series B		—		304		(304	)
Issue of share capital		7,596		304		79,530
IPO costs		—		—		(413	)
Balance at December 31, 2017		31,811		1,272		79,236

Other Reserves (Tables)

12 Months Ended

Dec. 31, 2017

Other Reserves

Disclosure Of Other Reserves [Line Items]

Schedule of Other Reserves

	2017			2016			2015
	(in thousands)
	£			£			£
Own share reserve		(339	)		(339	)		(339	)
Foreign currency translation reserve		(11	)		(3	)		(1	)
Capital reserve		42,466			—			—
Share option reserve
Balance at beginning of year		4,406			3,291			2,506
Share-based payments		11,731			1,132			785
Exercise of share options		(180	)		(17	)		—
Lapse of share options		(2	)		—			—
Balance at end of year		15,955			4,406			3,291
Total other reserves		58,071			4,064			2,951

Share-based Payments (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]

Summary of Share Options and Weighted Average Exercise Prices

Share options and weighted average exercise prices are as follows for the reporting periods presented:

	Number of shares			Weighted average exercise price per share
				£
Outstanding at January 1, 2015		2,873,747			0.26
Granted		—			—
Forfeited		—			—
Exercised		—			—
Outstanding at December 31, 2015		2,873,747			0.26
Granted		507,690			3.81
Forfeited		—			—
Cancelled		(50,000	)		2.95
Exercised¹		(83,250	)		2.40
Outstanding at December 31, 2016		3,248,187			0.72
Granted		1,500,815			2.58
Forfeited		(7,500	)		0.16
Cancelled		—			—
Exercised²		(30,000	)		4.00
Outstanding at December 31, 2017		4,711,502			1.29
Vested and exercisable at December 31, 2017³		4,030,833			0.36
Vested and exercisable at December 31, 2016		2,841,419			0.36
Vested and exercisable at December 31, 2015		2,262,500			0.23

1. The weighted average share price at the date of exercise of these options was £9.00

2. The weighted average share price at the date of exercise of these options was £10.43

3. Share options granted to a non-executive director in 2016 are not included in these calculations. These share options were exercised and will only be fully unencumbered after a period of four years from the grant date.

Summary of Principal Assumptions used for Valuation of Share Options

The following principal assumptions were used in the valuation for the 2016 share options.

		Options granted on
		Jun 30, 2016				Aug 22, 2016				Aug 22, 2016				Aug 22, 2016				Aug 22, 2016
Vesting dates		Oct 12, 2016				Aug 22, 2016				Aug 22, 2016				Dec 07, 2016				Mar 01, 2017
		Oct 12, 2017				Aug 22, 2017				Aug 22, 2017				Dec 07, 2017				Mar 01, 2018
		Oct 12, 2018												Dec 07, 2018				Mar 01, 2019
		Oct 12, 2019												Dec 07, 2019				Mar 01, 2020
Volatility			69.05	%			67.92	%			67.92	%			67.92	%			67.92	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.11	%			0.07	%			0.07	%			0.07	%			0.07	%
Fair value of option at grant date	£		4.02		£		6.15		£		6.01		£		5.74		£		5.62
Fair value of share at grant date	£		6.97		£		8.61		£		8.61		£		8.61		£		8.61
Exercise price at date of grant	£		4.00		£		2.80		£		3.00		£		3.40		£		3.60
Lapse date		June 30, 2026				Aug 22, 2026				Aug 22, 2026				Aug 22, 2026				Aug 22, 2026
Expected option life (years)			2.50				2.36				2.36				2.36				2.36
Number of options granted			270,690				12,500				37,500				25,000				11,250
		Options granted on
		Aug 22, 2016				Aug 22, 2016				Oct 28, 2016				Oct 28, 2016				Dec 12, 2016
Vesting dates		May 01, 2017				May 16, 2017				Oct 28, 2017				Oct 28, 2016				Dec 12, 2017
		May 01, 2018				May 16, 2018				Oct 28, 2018				Oct 28, 2017				Dec 12, 2018
		May 01, 2019				May 16, 2019				Oct 28, 2019				Oct 28, 2018				Dec 12, 2019
		May 01, 2020				May 16, 2020				Oct 28, 2020								Dec 12, 2020
Volatility			67.92	%			67.92	%			68.38	%			68.38	%			60.92	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.07	%			0.07	%			0.28	%			0.28	%			0.06	%

Fair value of option at grant date	£		5.62		£		5.62		£		5.85		£		5.46		£		5.22
Fair value of share at grant date	£		8.61		£		8.61		£		8.76		£		8.76		£		9.19
Exercise price at date of grant	£		3.60		£		3.60		£		3.40		£		4.00		£		4.00
Lapse date		Aug 22, 2026				Aug 22, 2026				Oct 28, 2026				Oct 28, 2026				Dec 12, 2026
Expected option life (years)			2.36				2.36				2.17				2.17				0.50
Number of options granted			5,000				37,500				12,500				50,000				45,750

The following principal assumptions were used in the valuation for share 2017 options.

		May 16, 2017				Sep 13, 2017				Sep 14, 2017				Sep 14, 2017				Sep 14, 2017
Vesting dates		Oct 28, 2017				Sep 13, 2018				Mar 06, 2018				Sep 14, 2018				Sep 14, 2018
		Oct 28, 2018				Sep 13, 2019				Mar 06, 2019				Sep 14, 2019				Sep 14, 2019
		Oct 28, 2019				Sep 13, 2020				Mar 06, 2020				Sep 14, 2020				Sep 14, 2020
		Oct 28, 2020				Sep 13, 2021				Mar 06, 2012				Sep 14, 2021				Sep 14, 2021
Volatility			66.56	%			66.94	%			66.97	%			66.97	%			66.97	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.12	%			0.27	%			0.36	%			0.36	%			0.36	%
Fair value of option at grant date	£		7.70		£		6.15		£		6.08		£		6.08		£		6.08
Fair value of share at grant date	£		11.08		£		10.43		£		10.34		£		10.34		£		10.34
Exercise price at date of grant	£		4.00		£		5.40		£		5.40		£		5.40		£		5.40
Lapse date		May 16, 2027				Sep 13, 2027				Sep 14, 2027				Sep 14, 2027				Sep 14, 2027
Expected option life (years)			2.63				2.30				2.30				2.30				2.30
Number of options granted			23,250				14,690				25,000				110,310				12,500

		Sep 14, 2017				Sep 15, 2017				Sep 15, 2017				Sep 27, 2017				Sep 27, 2017
Vesting dates		Sep 14, 2018				Sep 15, 2018				Sep 15, 2017				Mar 20, 2018				Sep 27, 2018
		Sep 14, 2019				Sep 15, 2019								Mar 20, 2019				Sep 27, 2019
		Sep 14, 2020				Sep 15, 2020								Mar 20, 2020				Sep 27, 2020
		Sep 14, 2021				Sep 15, 2021								Mar 20, 2021				Sep 27, 2021
Volatility			66.97	%			67.02	%			67.02	%			67.11	%			67.11	%
Dividend yield			0	%			0	%			0	%			0	%			0	%
Risk-free investment rate			0.36	%			0.44	%			0.44	%			0.47	%			0.47	%
Fair value of option at grant date	£		3.90		£		6.76		£		10.11		£		4.36		£		4.36
Fair value of share at grant date	£		10.34		£		10.15		£		10.15		£		11.19		£		11.19
Exercise price at date of grant	£		10.80		£		4.00		£		0.04		£		11.19		£		11.19
Lapse date		Sep 14, 2027				Sep 15, 2027				Sep 15, 2027				Sep 27, 2027				Sep 27, 2027
Expected option life (years)			2.30				2.29				2.29				2.26				2.26
Number of options granted			25,000				45,750				1,028,533				37,500				178,282

Related Party Disclosures (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Compensation of Key Management Personnel

Compensation of key management personnel of the Company

	2017		2016		2015
	(in thousands)
	£		£		£
Short-term employee benefits		1,184		1,053		647
Severance payments		—		150		—
Pension and other benefits		56		47		77
Share-based payments		11,230		731		726
		12,470		1,981		1,450

Cash, Cash Equivalents And Short-Term Deposits (Tables)

12 Months Ended

Dec. 31, 2017

Cash And Cash Equivalents [Abstract]

Schedule of Cash, Cash Equivalents and Short-Term Deposits

	2017		2016
	(in thousands)
	£		£
Cash and cash equivalents		86,703		19,990

Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure Of Financial Instruments [Abstract]

Schedule of Financial Assets Subject to Fixed or Variable Interest Rates

Financial assets subject to fixed or variable interest rates are as follows:

	2017		2016
	(in thousands)
	Carrying amount		Carrying amount
	£		£
Financial assets at fixed rates
Cash and cash equivalents		51,745		5,000
Financial assets at variable rates
Cash and cash equivalents		13,708		14,433

Summary of Financial Assets and Liabilities in Foreign Currencies

Financial assets and liabilities in foreign currencies are as follows:

	2017		2016
	(in thousands)
	Carrying amount		Carrying amount
	£		£
Financial assets
Prepayments, accrued income and other receivables		2,656		3,068
Current income tax receivable		18		54
Cash and cash equivalents		72,645		251

Financial liabilities
Trade payables		148		126
Payroll taxes and social security		3		—
Accrued expenditure		468		167

General Information- Additional Information (Details)	12 Months Ended
General Information- Additional Information (Details)	Dec. 31, 2017 Subsidiary
Disclosure Of General Information About Financial Statements [Abstract]
Name of reporting entity	NuCana plc
Description of nature of entity's operations	clinical-stage biopharmaceutical company developing a portfolio of new medicines to treat cancer
Explanation of change in name of reporting entity	On August 29, 2017 the Company re-registered as a public limited company and changed its name from NuCana BioMed Limited to NuCana plc.
Domicile of entity	United Kingdom
Country of incorporation	England and Wales
Name of subsidiary	NuCana, Inc. and NuCana BioMed Trustee Company Limited
Number of subsidiaries	2

Significant Accounting Policies - Additional Information (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2017 GBP (£) Segment	Dec. 31, 2016 GBP (£)	Dec. 31, 2015 GBP (£)	Dec. 31, 2014 GBP (£)
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents \| £	£ 86,703	£ 19,990	£ 14,112	£ 18,761
Number of operating segment \| Segment	1
Cash at banks maturity period	Less than three months
Certificates of deposits maturity period	Greater than three months but less than one year
Maximum
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Income tax credit, cash rebate percentage	33.35%
Patents
Disclosure Of Summary Of Significant Accounting Policies [Line Items]
Patent life	20 years

Significant Accounting Policies - Summary of Depreciation on Property, Plant and Equipment Over Their Expected Useful Economic Life (Details)	12 Months Ended
	Dec. 31, 2017
Office and Computer Equipment
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation method	Straight-line
Depreciation rate	3 years
Fixtures and Fittings
Disclosure Of Property Plant And Equipment [Line Items]
Depreciation method	Straight-line
Depreciation rate	5 years

Significant Accounting Policies - Summery of Amortization Method and Amortization Period for the Principal Categories of Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2017
Patents
Disclosure Of Intangible Assets [Line Items]
Amortization method	Straight-line
Amortization rate	20 years
Computer Software
Disclosure Of Intangible Assets [Line Items]
Amortization method	Straight-line
Computer Software \| Minimum [Member]
Disclosure Of Intangible Assets [Line Items]
Amortization rate	3 years
Computer Software \| Maximum
Disclosure Of Intangible Assets [Line Items]
Amortization rate	5 years

Staff Costs - Disclosure of Employee Benefit Expense (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Staff Costs [Line Items]
Share-based payments	£ 11,700	£ 1,100	£ 800
Total employee benefit expense	14,979	3,784	2,569
Research and Development Expenses
Staff Costs [Line Items]
Wages and salaries	2,391	2,241	1,366
Social security costs	272	205	151
Pension costs and other benefits	119	102	92
Share-based payments	8,868	1,027	709
Total employee benefit expense	11,650	3,575	2,318
Administrative Expenses
Staff Costs [Line Items]
Wages and salaries	408	84	108
Social security costs	37	13	52
Pension costs and other benefits	21	7	15
Share-based payments	2,863	105	76
Total employee benefit expense	£ 3,329	£ 209	£ 251

Staff Costs - Schedule of Average Number of Staff Employed Under Contracts of Service Explanatory (Details) - Employee	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Staff Costs [Line Items]
Average number of staff employed under contracts of service	20	16	12
Research and Development Expenses
Staff Costs [Line Items]
Average number of staff employed under contracts of service	18	15	11
Administrative Expenses
Staff Costs [Line Items]
Average number of staff employed under contracts of service	2	1	1

Staff Costs - Summary of Directors' Remuneration and Other Benefits (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Staff Costs [Line Items]
Directors’ remuneration	£ 793	£ 549	£ 526
Pension and other benefits	37	34	77
Directors’ remuneration & other benefits	830	583	603
Highest Paid Director
Staff Costs [Line Items]
Directors’ remuneration	601	378	318
Pension and other benefits	33	29	61
Directors’ remuneration & other benefits	£ 634	£ 407	£ 379

Staff Costs - Additional Information (Details) - Director	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Classes Of Employee Benefits Expense [Abstract]
Number of directors, exercised share options	0	1	0

Basic and Diluted Loss Per Share - Schedule of Basic and Diluted Loss Per Share (Details) - GBP (£) £ / shares in Units, £ in Thousands, shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Loss for the year	£ (23,085)	£ (6,049)	£ (5,332)
Basic and diluted weighted average number of shares	26,069	24,107	24,101
Basic and diluted loss per share	£ (0.89)	£ (0.25)	£ (0.22)

IPO Related Expenses - Summary of Initial Public Offering Related Expenses (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2017 GBP (£)
Disclosure Of Initial Public Offering Related Expenses [Abstract]
IPO related expenses	£ 1,794

Capital Commitments and Contingencies - Summary of Capital Commitments (Details) £ in Thousands	Dec. 31, 2017 GBP (£)
Capital Commitments [Abstract]
Future capital expenditure contracted but not provided for	£ 147

Capital Commitments and Contingencies - Additional Information (Details) - GBP (£)		12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure - Capital Commitments and Contingencies - Additional Information (Details) [Line Items]
Operating lease expense				£ 200,000	£ 200,000	£ 100,000
Cardiff University \| Scenario Forecast
Capital Commitments [Line Items]
Payments for work performed by sub-licensee		£ 292,607	£ 292,607
Cardiff University \| Renewing Significant Commitments
Capital Commitments [Line Items]
Research, collaboration and license agreement additional term	2 years
Cardiff Consultants
Capital Commitments [Line Items]
Milestone payments for sub-licensee				1,875,000
Cardiff ProTides
Capital Commitments [Line Items]
Milestone payments for sub-licensee				£ 4,500,000

Capital Commitments and Contingencies - Future Minimum Rentals Payable Under Non-cancellable Operating Leases (Details) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Finance Lease And Operating Lease By Lessee [Line Items]
Future minimum rentals payable under non-cancellable operating leases	£ 323	£ 254	£ 166
Not Later than 1 Year
Disclosure Of Finance Lease And Operating Lease By Lessee [Line Items]
Future minimum rentals payable under non-cancellable operating leases	158	192	£ 166
Later than 1 Year and Not Later than 5 Years
Disclosure Of Finance Lease And Operating Lease By Lessee [Line Items]
Future minimum rentals payable under non-cancellable operating leases	£ 165	£ 62

Income Tax Credit - Schedule of Major Components of Income Tax (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current tax:
Total current tax	£ 2,291	£ 2,116	£ 1,176
Deferred tax:
In respect of prior years, deferred tax	1,007	333	430
Total deferred tax	110
Income tax credit	2,401	2,116	1,176
Current income tax receivable	4,225	2,195	1,182
Deferred tax	81
U.K.
Current tax:
In respect of current year	2,298	1,906	1,182
In respect of prior years	3	235
Deferred tax:
Current income tax receivable	4,207	2,141	1,182
U.S.
Current tax:
In respect of current year	(10)	(28)	(6)
In respect of prior years		3
Deferred tax:
In respect of current year, deferred tax	61
In respect of prior years, deferred tax	49
Current income tax receivable	18	£ 54
Deferred tax	£ 81
Current income tax payable			£ 6

Income Tax Credit - Schedule of Reconciliation of Accounting Loss per Statement of Operations (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Abstract]
Loss before tax	£ (25,486)	£ (8,165)	£ (6,508)
Tax on loss at standard U.K. tax rate	(4,906)	(1,633)	(1,318)
Expenses not deductible	3,899	1,301	888
Deduction for R&D	(3,051)	(2,629)	(1,650)
Losses surrendered for R&D tax credit	3,051	2,629	1,650
Deferred tax - PY adjustment	(49)	24	6
Overseas tax payable	10
R&D tax credit— U.S.	(61)
R&D tax credit—current year	(2,298)	(1,906)	(1,182)
R&D tax credit—prior years	(3)	(235)
Deferred tax asset not recognized	1,007	333	430
Income tax credit	£ (2,401)	£ (2,116)	£ (1,176)

Income Tax Credit - Schedule of Reconciliation of Accounting Loss per Statement of Operations (Parenthetical) (Details)			12 Months Ended
	Apr. 01, 2017	Oct. 26, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.K.
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Line Items]
Applicable tax rate	19.00%	20.00%	19.25%	20.00%	20.25%

Income Tax Credit - Additional Information (Details) - GBP (£) £ in Millions				12 Months Ended
	Apr. 01, 2020	Apr. 01, 2017	Oct. 26, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items]
Temporary differences and cumulative carry forward tax losses for which deferred tax has not been recognized				£ 40.7	£ 32.5	£ 10.1
U.K.
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items]
Applicable tax rate		19.00%	20.00%	19.25%	20.00%	20.25%
Scenario Forecast \| U.K.
Disclosure Of Temporary Difference Unused Tax Losses And Unused Tax Credits [Line Items]
Applicable tax rate	17.00%
Applicable tax rate initially declared	18.00%
Percentage of change in tax rate	1.00%

Intangible Assets - Summary of Intangible Assets (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	£ 1,377
Ending Balance	1,938	£ 1,377
Cost
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	1,543	1,004
Additions	725	539
Ending Balance	2,268	1,543
Accumulated Amortization
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	166	77
Charge for the year	164	89
Ending Balance	330	166
Patents
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	1,370
Ending Balance	1,813	1,370
Patents \| Cost
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	1,533	1,004
Additions	582	529
Ending Balance	2,115	1,533
Patents \| Accumulated Amortization
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	163	77
Charge for the year	139	86
Ending Balance	302	163
Computer Software
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	7
Ending Balance	125	7
Computer Software \| Cost
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	10
Additions	143	10
Ending Balance	153	10
Computer Software \| Accumulated Amortization
Disclosure Of Intangible Assets [Line Items]
Beginning Balance	3
Charge for the year	25	3
Ending Balance	£ 28	£ 3

Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2015	£ 18
At December 31, 2016	358	£ 18
Cost
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2015	52	37
Additions	370	15
Disposals		(1)
Effect of foreign currency exchange differences		1
At December 31, 2016	422	52
Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2015	34	23
Charge for the year	30	12
Disposals		(1)
At December 31, 2016	64	34
Office and Computer Equipment
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2015	18
At December 31, 2016	91	18
Office and Computer Equipment \| Cost
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2015	52	37
Additions	90	15
Disposals		(1)
Effect of foreign currency exchange differences		1
At December 31, 2016	142	52
Office and Computer Equipment \| Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2015	34	23
Charge for the year	17	12
Disposals		(1)
At December 31, 2016	51	£ 34
Fixtures and Fittings
Disclosure Of Property Plant And Equipment [Line Items]
At December 31, 2016	267
Fixtures and Fittings \| Cost
Disclosure Of Property Plant And Equipment [Line Items]
Additions	280
At December 31, 2016	280
Fixtures and Fittings \| Depreciation
Disclosure Of Property Plant And Equipment [Line Items]
Charge for the year	13
At December 31, 2016	£ 13

Prepayments, Accrued Income and Other Receivables - Summary of Prepayments, Accrued Income and Other Receivables Explanatory (Details) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Prepayments Accrued Income And Other Receivables [Abstract]
Prepayments—manufacturing and clinical	£ 1,979	£ 3,050
Prepayments—other	522	173
Accrued income	67	21
VAT	473	198
Other receivables	9	192
Prepayments, accrued income and other receivables	£ 3,050	£ 3,634

Share Capital and Share Premium - Summary of Share Capital and Share Premium (Details) - GBP (£) £ in Thousands	Dec. 31, 2017	Aug. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Classes Of Share Capital [Line Items]
Share capital	£ 1,272		£ 663	£ 659
Share premium	79,236		42,770	42,574
Share capital and share premium	£ 80,508		£ 43,433	£ 43,233
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)	31,811,000	30,000	24,185,000	24,102,000
Ordinary Shares
Disclosure Of Classes Of Share Capital [Line Items]
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)	31,811,000		6,239,000	6,156,000
Founder Ordinary 1 Shares
Disclosure Of Classes Of Share Capital [Line Items]
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)			1,000,000	1,000,000
Founder Ordinary 2 Shares
Disclosure Of Classes Of Share Capital [Line Items]
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)			1,000,000	1,000,000
Series A Shares
Disclosure Of Classes Of Share Capital [Line Items]
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)			7,483,000	7,483,000
Series B Shares
Disclosure Of Classes Of Share Capital [Line Items]
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each)			8,463,000	8,463,000

Share Capital and Share Premium - Summary of Share Capital and Share Premium (Parenthetical) (Details) - £ / shares	Dec. 31, 2017	Sep. 14, 2017	Dec. 31, 2016	Dec. 31, 2015
Ordinary Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	£ 0.04	£ 0.01	£ 0.01	£ 0.01
Founder Ordinary 1 Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.01	0.01
Founder Ordinary 2 Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.01	0.01
Series A Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.01	0.01
Series B Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	£ 0.001	£ 0.001

Share Capital and Share Premium - Additional Information (Details)			12 Months Ended
	Sep. 14, 2017 GBP (£) £ / shares shares	Jun. 29, 2017 GBP (£)	Dec. 31, 2017 £ / shares shares	Oct. 02, 2017 $ / shares shares	Aug. 31, 2017 shares	Dec. 31, 2016 £ / shares shares	Dec. 31, 2015 £ / shares shares
Disclosure Of Classes Of Share Capital [Line Items]
Number of shares outstanding \| shares	24,214,641
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares			31,811,000		30,000	24,185,000	24,102,000
Description of conversion of series b shares into ordinary shares			Following the one-for-four reverse share split, for the purpose of facilitating a conversion of each Series B share (nominal value £0.004 per share), into an ordinary share (nominal value £0.04 per share), the company allotted to holders of Series B shares an additional nine Series B shares for each Series B share held. Subjected to and conditional upon this allotment, every 10 Series B shares of £0.004 were consolidated into a single Series B share of £0.04. Each Series B share of £0.04 was then automatically converted into one ordinary share of £0.04.
Reserve to fund allotment of additional shares from reverse share split \| £	£ 304,650
Ordinary Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	£ 0.01		£ 0.04			£ 0.01	£ 0.01
Par value per share after reverse share split	0.04
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares			31,811,000			6,239,000	6,156,000
Par value per share after stock conversion	0.04
Founder Ordinary 1 Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.01		£ 0.01
Par value per share after reverse share split	0.04
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares						1,000,000	1,000,000
Proportion of voting rate			5.00%
Founder Ordinary 2 Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.01		£ 0.01
Par value per share after reverse share split	0.04
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares						1,000,000	1,000,000
Proportion of voting rate			5.00%
Series A Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.01		£ 0.01
Par value per share after reverse share split	0.04
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares						7,483,000	7,483,000
Series B Shares
Disclosure Of Classes Of Share Capital [Line Items]
Par value per share	0.001		£ 0.001
Par value per share after reverse share split	0.004
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares						8,463,000	8,463,000
Par value per share after stock conversion	£ 0.04
IPO
Disclosure Of Classes Of Share Capital [Line Items]
Ordinary shares of £0.04 each (2016 and 2015: £0.01 each) \| shares				7,596,505,000
Number of share issued price per share \| $ / shares				$ 15.00
Share Premium
Disclosure Of Classes Of Share Capital [Line Items]
Reduction of capital by reducing share premium \| £		£ 42,500,000

Share Capital and Share Premium - Summary of Movement in the Share Capital (Details) £ in Thousands	12 Months Ended
	Dec. 31, 2017 GBP (£) shares	Dec. 31, 2016 GBP (£) shares
Disclosure Of Classes Of Share Capital [Line Items]
Beginning balance	£ 25,241	£ 29,960
Beginning balance, Shares \| shares	24,185,000	24,102,000
Issue of share capital	£ 79,834
Issue of shares on exercise of options, Shares \| shares		83,000
Exercise of share options	£ 120	£ 200
Exercise of share options, Shares \| shares	30,000	83,250
Issue of share capital, Shares \| shares	7,596,000
IPO costs	£ (413)
Ending balance	£ 93,420	£ 25,241
Ending balance, Shares \| shares	31,811,000	24,185,000
Share Capital
Disclosure Of Classes Of Share Capital [Line Items]
Beginning balance	£ 663	£ 659
Issue of share capital	304	4
Exercise of share options	1	4
Bonus issue to series B shareholders	304
Ending balance	1,272	663
Share Premium
Disclosure Of Classes Of Share Capital [Line Items]
Beginning balance	42,770	42,574
Issue of share capital	79,530	196
Reduction in share premium	(42,466)
Exercise of share options	119	196
Bonus issue to series B shareholders	(304)
IPO costs	(413)
Ending balance	£ 79,236	£ 42,770

Other Reserves - Schedule of Other Reserves (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share option reserve
Total other reserves	£ 58,071	£ 4,064
Other Reserves
Disclosure Of Other Reserves [Line Items]
Own share reserve	(339)	(339)	£ (339)
Foreign currency translation reserve	(11)	(3)	(1)
Capital reserve	42,466
Share option reserve
Balance at beginning of year	4,406	3,291	2,506
Share-based payments	11,731	1,132	785
Exercise of share options	(180)	(17)
Lapse of share options	(2)
Balance at end of year	15,955	4,406	3,291
Total other reserves	£ 58,071	£ 4,064	£ 2,951

Other Reserves - Additional Information (Details)	Dec. 31, 2017 shares
Disclosure Of Other Reserves [Abstract]
Number of shares purchased by employee benefit trust under share options plan	500,000

Share-based Payments - Additional Information (Details)			12 Months Ended
Share-based Payments - Additional Information (Details)	Dec. 12, 2016 GBP (£) shares	Jun. 30, 2016	Dec. 31, 2017 GBP (£) shares yr Plan	Dec. 31, 2016 GBP (£) shares yr	Dec. 31, 2015 GBP (£) shares yr
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Number of equity settled share-based payment plans \| Plan			3
Description of vesting requirements for share-based payment			1) options granted to a director, under which a third of the options granted vested immediately with the remaining two-thirds vesting each subsequent year; and 2) options granted to three employees, under which two thirds of the options vested immediately with the remaining third vesting in 2017.
Number of share options granted in share-based payment arrangement \| shares			1,500,815	507,690	0
Weighted average remaining contractual life of outstanding share options \| yr			4.49	6.66	6.18
Employee remuneration expense, related to equity-settled share-based payments \| £			£ 11,700,000	£ 1,100,000	£ 800,000
U.K. Share-based Payment Plans
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Share options granted, vesting period			4 years	4 years
Stock Option Plan (U.S. Sub-Plan)
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Share options granted, vesting period		4 years	4 years
Number of share options granted in share-based payment arrangement \| shares	45,750
Share options exercise price per share \| £	£ 4.00
Share option exercised date	Dec. 29, 2016
Stock Option Plan (U.S. Sub-Plan) \| Maximum
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Term of share options exercised	6 months
2016 Share Options
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Discount rate used to compute present value				25.00%
2017 Share Options
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]
Discount rate used to compute present value			25.00%

Share-based Payments - Summary of Share Options and Weighted Average Exercise Prices (Details)	12 Months Ended
	Dec. 31, 2017 GBP (£) shares	Dec. 31, 2016 GBP (£) shares	Dec. 31, 2015 GBP (£) shares
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]
Number of shares, Outstanding, Beginning balance \| shares	3,248,187	2,873,747	2,873,747
Number of share options granted in share-based payment arrangement \| shares	1,500,815	507,690	0
Number of shares, Forfeited \| shares	(7,500)
Number of shares, Cancelled \| shares		(50,000)
Number of shares, Exercised \| shares	(30,000)	(83,250)
Number of shares, Outstanding, Ending balance \| shares	4,711,502	3,248,187	2,873,747
Number of shares, Vested and exercisable, Ending balance \| shares	4,030,833	2,841,419	2,262,500
Weighted average exercise price per share, Outstanding, Beginning balance \| £	£ 0.72	£ 0.26	£ 0.26
Weighted average exercise price per share, Granted \| £	2.58	3.81	0
Weighted average exercise price per share, Forfeited \| £	0.16
Weighted average exercise price per share, Cancelled \| £		2.95
Weighted average exercise price per share, Exercised \| £	4.00	2.40
Weighted average exercise price per share, Outstanding, Ending balance \| £	1.29	0.72	0.26
Weighted average exercise price per share, Vested and exercisable, Ending balance \| £	£ 0.36	£ 0.36	£ 0.23

Share-based Payments - Summary of Share Options and Weighted Average Exercise Prices (Parenthetical) (Details) - GBP (£)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Abstract]
Weighted average share price for share options at date of exercise	£ 10.43	£ 9.00
Vesting period of share options exercised	4 years

Related Party Disclosures - Summary of Compensation of Key Management Personnel (Details) - GBP (£) £ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Key Management Personnel Compensation [Abstract]
Short-term employee benefits	£ 1,184	£ 1,053	£ 647
Severance payments		150
Pension and other benefits	56	47	77
Share-based payments	11,230	731	726
Key management personnel compensation	£ 12,470	£ 1,981	£ 1,450

Cash, Cash Equivalents and Short-Term Deposits - Schedule of Cash, Cash Equivalents and Short-Term Deposits (Detail) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	£ 86,703	£ 19,990	£ 14,112	£ 18,761

Financial Instruments - Additional Information (Details) - GBP (£)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Financial Instruments [Line Items]
Cash and cash equivalents	£ 86,703,000	£ 19,990,000	£ 14,112,000	£ 18,761,000
Interest Rate Risk
Disclosure Of Financial Instruments [Line Items]
Cash and cash equivalents	£ 86,700,000	20,000,000
Increase in interest rates	0.50%
Increase in interest income on cash and cash equivalents	£ 327,261	97,165
Currency Risk
Disclosure Of Financial Instruments [Line Items]
Cash and cash equivalents	£ 72,645,000	251,000
Percentage of increase in foreign exchange rate	1.00%
Reduction in financial assets and liabilities in foreign currencies	£ (747,004)	£ (30,807)

Financial Instruments - Schedule of Financial Assets Subject to Fixed or Variable Interest Rates (Details) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	£ 86,703	£ 19,990	£ 14,112	£ 18,761
Financial Assets at Fixed Rates
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	51,745	5,000
Financial Assets at Variable Rates
Disclosure Of Financial Instruments By Type Of Interest Rate [Line Items]
Cash and cash equivalents	£ 13,708	£ 14,433

Financial Instruments - Summary of Financial Assets and Liabilities in Foreign Currencies (Details) - GBP (£) £ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial assets
Prepayments, accrued income and other receivables	£ 3,050	£ 3,634
Current income tax receivable	4,225	2,195	£ 1,182
Cash and cash equivalents	86,703	19,990	£ 14,112	£ 18,761
Financial liabilities
Trade payables	1,120	728
Payroll taxes and social security	157	61
Accrued expenditure	1,640	1,184
Currency Risk
Financial assets
Prepayments, accrued income and other receivables	2,656	3,068
Current income tax receivable	18	54
Cash and cash equivalents	72,645	251
Financial liabilities
Trade payables	148	126
Payroll taxes and social security	3
Accrued expenditure	£ 468	£ 167