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Financial instruments and risk management - Fair Value Estimation, Additional Information (Details) - GBP (£)
£ in Millions
12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Disclosure Of Fair Value Measurement Of Liabilities [Line Items]        
Total equity £ 49,995 £ 52,934 £ 75,710 £ 67,401
Hedging reserve        
Disclosure Of Fair Value Measurement Of Liabilities [Line Items]        
Total equity (84) (194) £ (327) £ (363)
Cash flow hedges        
Disclosure Of Fair Value Measurement Of Liabilities [Line Items]        
Cash flow hedge reserve (268) (362)    
Cash flow hedges | Currency Risk        
Disclosure Of Fair Value Measurement Of Liabilities [Line Items]        
Cash flow hedge reserve £ 54 £ 77    
Long-term Borrowings | Net investment hedges (non-derivative related)        
Disclosure Of Fair Value Measurement Of Liabilities [Line Items]        
Nominal amount of hedging instrument 363 380    
Changes in fair value used for calculating hedge ineffectiveness £ 17 £ 9