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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