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Employee benefit obligations - Additional Information (Details) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Disclosure of defined benefit plans [line items]      
Employer contributions (for funded plans) and benefit payments (for unfunded plans) £ 20 £ 20 £ 24
Employer contributions (for funded plans) and benefit payments (for unfunded plans), expected in 2025 £ 18    
Current pensioners (at age 65) – male      
Disclosure of defined benefit plans [line items]      
Actuarial assumption of life expectancy 21 years 9 months 18 days 21 years 9 months 18 days  
Current pensioners (at age 65) – female      
Disclosure of defined benefit plans [line items]      
Actuarial assumption of life expectancy 23 years 7 months 6 days 23 years 7 months 6 days  
Future pensioners (current age 45) – male      
Disclosure of defined benefit plans [line items]      
Actuarial assumption of life expectancy 23 years 4 months 24 days 23 years 6 months  
Future pensioners (current age 45) – female      
Disclosure of defined benefit plans [line items]      
Actuarial assumption of life expectancy 25 years 1 month 6 days 25 years 2 months 12 days