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Costs of services and general and administrative costs - Additional Information (Detail) - GBP (£)
£ in Millions
 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Disclosure Of Information About Operating Cost [line items]    
Restructuring and transformation costs £ 34.3 £ 17.9 [1]
Litigation settlement 21.7 [1]
Impairment charge on intangible assets 3.9 17.1
Restructuring costs in relation to Covid-19 19.7 39.3 [1]
Impairment of goodwill £ 2,812.9 [1]
Covid-19 Pandemic    
Disclosure Of Information About Operating Cost [line items]    
Restructuring costs in relation to Covid-19 19.7  
Investor day in december two thousand and eighteen restructuring and transformation plan [member]    
Disclosure Of Information About Operating Cost [line items]    
IT Transformation Costs 8.1  
Restructuring costs in relation to the continuing restructuring plan £ 26.2  
[1] Figures have been restated as described in the accounting policies.