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Staff Costs - Additional Information (Details)
£ in Millions
12 Months Ended
Dec. 31, 2019
GBP (£)
Director
Dec. 31, 2018
GBP (£)
Director
Classes Of Employee Benefits Expense [Abstract]    
Number of directors, exercised share options | Director 1 1
Gain on exercise of option | £ £ 1.4 £ 4.2