v3.24.1.1.u2
Sensitivities - Schedule of sensitivities on areas of estimation uncertainty (Details) - GBP (£)
£ in Millions
12 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Other environment related provision | Change in the real discount rate    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in risk variable, percent 1.00% 1.00%
Change in risk variable, impact on income statement £ 173 £ 150
Change in risk variable, impact on net assets £ 173 150
Other environment related provision | Change in the real discount rate | Increase (decrease) due to voluntary changes in accounting policy    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in risk variable, impact on income statement   (9)
Change in risk variable, impact on net assets   £ (24)
Other environment related provision | Change in estimated future cash flows    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in risk variable, percent 20.00% 20.00%
Change in risk variable, impact on income statement £ 462 £ 354
Change in risk variable, impact on net assets 462 354
UK | Actuarial assumption of discount rates    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption 22 29
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 1,147 1,264
UK | Actuarial assumption of expected rates of inflation    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption 8 8
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 902 933
UK | Actuarial assumption of expected rates of salary increases    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase in actuarial assumption 4 4
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption 81 50
UK | Actuarial assumption of life expectancy after retirement    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption 2 2
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 402 441
UK | Pensions | Value of buy-in policies offset to actuarial assumption of discount rate    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in risk variable, impact on net assets (171) (188)
UK | Pensions | Value of buy-in policies offset to actuarial assumption of expected rates of inflation    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in risk variable, impact on net assets (150) (164)
UK | Pensions | Value of buy-in policies and longevity swap offset to actuarial assumption of life expectancy after retirement    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in risk variable, impact on net assets £ (126) £ (136)
UK | Pensions | Actuarial assumption of discount rates    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in actuarial assumption, percent 1.00% 1.00%
UK | Pensions | Actuarial assumption of expected rates of inflation    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in actuarial assumption, percent 1.00% 1.00%
UK | Pensions | Actuarial assumption of expected rates of salary increases    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase in actuarial assumption, percent 1.00% 1.00%
UK | Pensions | Actuarial assumption of life expectancy after retirement    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase in actuarial assumption, years 1 year 1 year
US | Actuarial assumption of discount rates    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption £ 18 £ 26
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 801 977
US | Actuarial assumption of expected rates of salary increases    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase in actuarial assumption 2 4
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption 37 57
US | Actuarial assumption of life expectancy after retirement    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption 2 3
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 288 344
US | Assumed US healthcare cost trend rates change    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase (decrease) in defined benefit plan expense due to reasonably possible increase (decrease) in actuarial assumption 18 24
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption £ 276 £ 324
US | Pensions | Actuarial assumption of discount rates    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in actuarial assumption, percent 1.00% 1.00%
US | Pensions | Actuarial assumption of expected rates of salary increases    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase in actuarial assumption, percent 1.00% 1.00%
US | Pensions | Actuarial assumption of life expectancy after retirement    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Increase in actuarial assumption, years 1 year 1 year
US | Pensions | Assumed US healthcare cost trend rates change    
Disclosure of sensitivity analysis for actuarial assumptions [line items]    
Change in actuarial assumption, percent 1.00% 1.00%