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Pensions - Investment strategy (Details) - GBP (£)
£ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Pensions    
Fair value of assets £ 78,406 £ 78,904
Fair value of liabilities 72,082 72,395
Interest rate Contracts    
Pensions    
Derivatives notional (£bn) 7,015,000 6,533,000
Fair value of assets 28,960 33,807
Fair value of liabilities £ 23,138 £ 27,424
Main Pension Scheme    
Pensions    
Percentage of total plan assets 90.00% 91.00%
Percentage of defined benefit obligation covered by buy-in insurance contracts 33.33%  
Equities 6.70% 6.80%
Index-linked bonds 23.60% 36.70%
Government bonds 9.90% 13.30%
Corporate and other bonds 18.50% 25.60%
Real estate 2.40% 4.50%
Derivatives 0.10% 2.70%
Buy-in insurance contracts 27.00%  
Cash and other assets 11.80% 10.40%
Total 100.00% 100.00%
Percentage of fair value of derivatives included in plan assets for which counterparty has posted collateral 85.00%  
Maximum percentage of indirect investment by trustees 5.00%  
Main Pension Scheme | Quoted plan assets    
Pensions    
Equities 0.10% 0.10%
Index-linked bonds 23.60% 36.70%
Government bonds 9.90% 13.30%
Corporate and other bonds 14.40% 19.20%
Total 48.00% 69.30%
Main Pension Scheme | Unquoted plan assets    
Pensions    
Equities 6.60% 6.70%
Corporate and other bonds 4.10% 6.40%
Real estate 2.40% 4.50%
Derivatives 0.10% 2.70%
Buy-in insurance contracts 27.00%  
Cash and other assets 11.80% 10.40%
Total 52.00% 30.70%
Main Pension Scheme | Swap    
Pensions    
Derivatives notional (£bn) £ 81,000 £ 81,000
Derivatives, amount contributed to fair value of plan assets 73 714
Main Pension Scheme | Inflation rate swaps    
Pensions    
Derivatives notional (£bn) 24,000 29,000
Fair value of assets 1,548 1,929
Fair value of liabilities 812 940
Main Pension Scheme | Interest rate Contracts    
Pensions    
Derivatives notional (£bn) 57,000 52,000
Fair value of assets 3,096 3,121
Fair value of liabilities 3,763 3,394
Main Pension Scheme | Currency forwards    
Pensions    
Derivatives notional (£bn) 8,000 13,000
Fair value of assets 60 235
Fair value of liabilities 130 34
Main Pension Scheme | Equity and bond put options    
Pensions    
Fair value of liabilities   4
Main Pension Scheme | Other    
Pensions    
Derivatives notional (£bn) 1,000 1,000
Fair value of assets 22 8
Fair value of liabilities £ 4 £ 20
AA section of the group pension fund    
Pensions    
Percentage of defined benefit obligation covered by buy-in insurance contracts 99.00%