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Pensions - Funding and contributions by the Group and most significant assumptions used for the Main Scheme (Details) - GBP (£)
£ in Millions
 12 Months Ended 24 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2022
Dec. 31, 2018
Pensions        
Fixed interest swap and guilt yields plus percentage 0.64%      
Life expectancy of male employees after retirement who are currently at age 60 27 years 8 months 12 days      
Life expectancy of female employees after retirement who are currently at age 60 29 years 4 months 24 days      
Life expectancy of male employees after retirement who are currently aged 40 28 years 10 months 24 days      
Life expectancy of female employees after retirement who are currently aged 40 30 years 8 months 12 days      
Main Pension Scheme        
Pensions        
Period of funding valuations (in years) 3 years      
Funding level, as percent   104.00%    
Pension liabilities   £ 49,000 £ 49,000  
Surplus as per actuarial assumptions   £ 2,000 2,000  
Average cost (as a percent)   49.00%    
Future contribution to funding if distribution to shareholders are made, capped amount       £ 1,500
Contributions Resulting From Ring Fencing Arrangements     £ 1,000  
Discount rate 4.80% 5.00% 5.00%  
Inflation assumption (RPI) 3.10% 3.20% 3.20%  
Rate of increase in salaries 1.80% 1.80% 1.80%  
Rate of increase in deferred pensions 3.20% 3.20% 3.20%  
Rate of increase in pensions in payment 2.40% 2.50% 2.50%  
Proportion of pension converted to a cash lump sum at retirement 18.00% 18.00% 18.00%  
Longevity at age 60 for current pensioners (years), males 26 years 9 months 18 days 27 years 3 months 18 days    
Longevity at age 60 for current pensioners (years), females 28 years 7 months 6 days 29 years 1 month 6 days    
Longevity at age 60 for future pensioners currently aged 40 (years), males 27 years 8 months 12 days 28 years 3 months 18 days    
Longevity at age 60 for future pensioners currently aged 40 (years), females 29 years 6 months 30 years 1 month 6 days    
Main Pension Scheme | Maximum | Distributions to shareholders of an equal amount are made [Member]        
Pensions        
Future contribution to funding if distribution to shareholders are made £ 500