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Costs of services and general and administrative costs - Additional Information (Details) - GBP (£)
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure Of Information About Operating Cost [line items]      
Credits relating to the release of excess provisions and other balances £ 16,900,000 £ 29,300,000 £ 19,300,000
Goodwill impairment 63,600,000 37,900,000 1,800,000
Amortisation and impairment of acquired intangible assets 727,900,000 62,100,000 97,800,000
Investment and other impairment charges/(reversals) 17,800,000 77,000,000.0 (42,400,000)
Impairments related to macro-economic factors   48,000,000.0  
Impairments related to capitalized configuration and customization costs   29,000,000.0  
Restructuring and transformation costs 195,500,000 218,800,000 175,400,000
Restructuring and transformation costs, IT Transformation Program 113,400,000 134,500,000 94,200,000
Restructuring and transformation costs in relation to the continuing restructuring plan 72,300,000 69,200,000 51,300,000
Property-related restructuring costs 232,500,000 18,000,000 0
Impairment loss, right of use assets 128,800,000    
Impairment charges included within restructuring costs 55,800,000    
Gains (losses) on disposals of investments (7,100,000) 36,300,000 10,600,000
Losses on disposals of investments   36,300,000 10,600,000
(Gains)/losses on disposal of investments and subsidiaries 7,100,000    
Gains (losses) on remeasurement of equity interests arising from change in scope of ownership 0 (66,500,000) 0
Gains on litigation settlements 11,000,000.0 0  
Associates      
Disclosure Of Information About Operating Cost [line items]      
Amortisation and impairment of acquired intangible assets   200,000  
Kantar      
Disclosure Of Information About Operating Cost [line items]      
Gains (losses) on disposals of investments (18,100,000)    
Astus Australia      
Disclosure Of Information About Operating Cost [line items]      
Gains (losses) on disposals of investments 11,000,000.0    
Russian Interest      
Disclosure Of Information About Operating Cost [line items]      
Losses on disposals of investments   63,100,000  
Res Publica      
Disclosure Of Information About Operating Cost [line items]      
(Gains)/losses on disposal of investments and subsidiaries   17,700,000  
Mutual Mobile      
Disclosure Of Information About Operating Cost [line items]      
(Gains)/losses on disposal of investments and subsidiaries   9,400,000  
XMKT      
Disclosure Of Information About Operating Cost [line items]      
Losses on disposals of investments     4,900,000
Imagina Spain      
Disclosure Of Information About Operating Cost [line items]      
Gains (losses) on remeasurement of equity interests arising from change in scope of ownership   (66,500,000) 0
Brand Names No Longer In Use      
Disclosure Of Information About Operating Cost [line items]      
Impairment loss recognised in profit or loss, intangible assets other than goodwill   1,400,000 47,900,000
Brand Names No Longer In Use | Wunderman Thompson      
Disclosure Of Information About Operating Cost [line items]      
Impairment loss recognised in profit or loss, intangible assets other than goodwill 430,800,000    
Brand Names No Longer In Use | Y&R Brands      
Disclosure Of Information About Operating Cost [line items]      
Impairment loss recognised in profit or loss, intangible assets other than goodwill 202,300,000    
Brand names      
Disclosure Of Information About Operating Cost [line items]      
Impairment loss recognised in profit or loss, intangible assets other than goodwill 650,100,000 1,400,000 47,900,000
ERP System      
Disclosure Of Information About Operating Cost [line items]      
Restructuring and transformation costs (52,300,000) (96,800,000) (62,200,000)
Transformation Costs      
Disclosure Of Information About Operating Cost [line items]      
Restructuring and transformation costs 38,300,000 0 0
Covid-19 Pandemic      
Disclosure Of Information About Operating Cost [line items]      
Restructuring Costs Due To Covid Nineteen Pandemic £ 9,800,000 £ 15,100,000 £ 29,900,000