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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% |  |  |  |  |

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