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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