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Share-based Payments - Summary of Share Options and Weighted Average Exercise Prices (Parenthetical) (Details) - £ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Weighted average share price for share options at date of exercise £ 4.63 £ 7.65 £ 18.23
Bottom Of Range [Member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Exercise price of outstanding share options 0.04    
Maximum [member]      
Disclosure Of Terms And Conditions Of Sharebased Payment Arrangement [Line Items]      
Exercise price of outstanding share options £ 18.05