v3.26.1
Sensitivities - Schedule of sensitivities on areas of estimation uncertainty (Details) - GBP (£)
£ in Millions
12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
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 £ 148 £ 155
Change in risk variable, impact on net assets £ 148 £ 155
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 £ 402 £ 413
Change in risk variable, impact on net assets 402 413
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 15 20
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 880 920
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 5 6
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 648 701
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 2 1
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption 37 52
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 0 0
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 317 320
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 (329) (288)
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 (235) (211)
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 £ (154) £ (109)
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 £ 16 £ 18
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 671 784
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 3
Increase (decrease) in defined benefit obligation due to reasonably possible increase in actuarial assumption 41 46
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 1 2
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption 137 181
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 15 19
Increase (decrease) in defined benefit obligation due to reasonably possible increase (decrease) in actuarial assumption £ 195 £ 245
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%