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Intangible assets - Additional Information (Details) - GBP (£)
£ in Millions
 6 Months Ended 12 Months Ended
Jul. 01, 2024
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets   £ 737 £ 850 £ 1,452
Amortisation and impairment of acquired intangible assets   93 728 62
Goodwill impairment   £ 237 63 38
Reduction in operating margin due to amount recovered   2.00%    
Imapirment charges due to amount recovered   £ 70    
Accumulated impairment losses        
Disclosure of detailed information about intangible assets [line items]        
Goodwill impairment   237 63 38
AKQA Group        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets   £ 491    
Management forecasts for a projection period   five years    
Impairment loss recognised in profit or loss, intangible assets other than goodwill   £ 158    
Goodwill impairment   237    
Global Integrated Agencies        
Disclosure of detailed information about intangible assets [line items]        
Impairment loss recognised in profit or loss, intangible assets other than goodwill   158 40 0
Public Relations        
Disclosure of detailed information about intangible assets [line items]        
Impairment loss recognised in profit or loss, intangible assets other than goodwill   12 0 4
Specialist Agencies        
Disclosure of detailed information about intangible assets [line items]        
Impairment loss recognised in profit or loss, intangible assets other than goodwill   £ 67 £ 23 34
First Step        
Disclosure of detailed information about intangible assets [line items]        
Assumed annual long-term growth rate   0.00% 0.00%  
Discount rate applied to cash flow projections   13.30% 14.70%  
Range of Pre-tax Discount Rates for Global Cash Generating Units        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections   12.30% 13.70%  
Range of Pre-tax Discount Rates for Geographic Regions        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections   1.00%    
Range of Pre-tax Discount Rates for Geographic Regions | Bottom of Range        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections   11.50% 12.60%  
Range of Pre-tax Discount Rates for Geographic Regions | Top of Range        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections   18.40% 28.40%  
Second Step        
Disclosure of detailed information about intangible assets [line items]        
Assumed annual long-term growth rate   2.00% 2.00%  
Management forecasts for a projection period   five years    
FVLCD of AKQA Group | AKQA Group        
Disclosure of detailed information about intangible assets [line items]        
Discount rate applied to cash flow projections   10.50%    
Brand names        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets   £ 83 £ 135  
Impairment loss recognised in profit or loss, intangible assets other than goodwill   21 650 £ 1
C&W Brands        
Disclosure of detailed information about intangible assets [line items]        
Amortisation and impairment of acquired intangible assets £ 20      
Customer-related intangibles        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets   50 108  
Other acquired intangible assets        
Disclosure of detailed information about intangible assets [line items]        
Other intangible assets   £ 83 £ 101