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Trade and other receivables - Additional Information (Detail) - GBP (£)
£ in Millions
 6 Months Ended 12 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Dec. 31, 2020
Disclosure of detailed information about financial instruments [Line Items]      
Allowance for bad and doubtful debts as a percentage of gross trade accounts receivables 1.40%   1.70%
Trade receivables [member]      
Disclosure of detailed information about financial instruments [Line Items]      
Credit to bad debt expense £ 10.6    
Bad debt expenses   £ 29.2 £ 40.8