v3.26.1
Pensions and other post-retirement benefits - Disclosure of asset allocations (Details) - GBP (£)
£ in Millions
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
UK | Pensions      
Disclosure of fair value of plan assets [line items]      
Equities £ 945 £ 839 £ 729
Property 403 451 679
Diversified alternatives 643 809 1,241
Bulk annuity policies 4,059 3,239 2,060
Longevity swap 0 0 (94)
Cash and cash equivalents 0 1 3
Other (5) (11) (5)
Fair value of plan assets 10,441 10,603 11,782
Pension fund investments in employer issued securities 0 0 44
Repurchase agreements 2,500 2,900 2,700
UK | Pensions | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 1,018 1,337 1,910
UK | Pensions | Government securities and liability-driven investments      
Disclosure of fair value of plan assets [line items]      
Debt instruments 3,378 3,938 5,259
UK | Pensions | Quoted      
Disclosure of fair value of plan assets [line items]      
Equities 842 716 576
Property 0 0 0
Diversified alternatives 412 381 669
Bulk annuity policies 0 0 0
Longevity swap 0 0 0
Cash and cash equivalents 0 1 3
Other 0 0 0
Fair value of plan assets 2,272 2,436 3,158
UK | Pensions | Quoted | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 1,018 1,338 1,910
UK | Pensions | Quoted | Government securities and liability-driven investments      
Disclosure of fair value of plan assets [line items]      
Debt instruments 0 0 0
UK | Pensions | Unquoted      
Disclosure of fair value of plan assets [line items]      
Equities 103 123 153
Property 403 451 679
Diversified alternatives 231 428 572
Bulk annuity policies 4,059 3,239 2,060
Longevity swap 0 0 (94)
Cash and cash equivalents 0 0 0
Other (5) (11) (5)
Fair value of plan assets 8,169 8,167 8,624
Investments in forestry funds 284 294 288
UK | Pensions | Unquoted | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 0 (1) 0
UK | Pensions | Unquoted | Government securities and liability-driven investments      
Disclosure of fair value of plan assets [line items]      
Debt instruments 3,378 3,938 5,259
US | Pensions      
Disclosure of fair value of plan assets [line items]      
Equities 848 887 1,323
Property 163 196 237
Diversified alternatives 357 384 556
Cash and cash equivalents 130 152 9
Other 8 1 1
Fair value of plan assets 4,608 5,180 5,320
US | Pensions | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 2,021 2,356 2,390
US | Pensions | Government securities      
Disclosure of fair value of plan assets [line items]      
Debt instruments 1,081 1,204 804
US | Pensions | Quoted      
Disclosure of fair value of plan assets [line items]      
Equities 0 0 99
Property 0 0 0
Diversified alternatives 0 0 54
Cash and cash equivalents 130 152 9
Other 4 (2) 1
Fair value of plan assets 2,437 2,842 2,510
US | Pensions | Quoted | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 1,682 1,955 1,987
US | Pensions | Quoted | Government securities      
Disclosure of fair value of plan assets [line items]      
Debt instruments 621 737 360
US | Pensions | Unquoted      
Disclosure of fair value of plan assets [line items]      
Equities 848 887 1,224
Property 163 196 237
Diversified alternatives 357 384 502
Cash and cash equivalents 0 0 0
Other 4 3 0
Fair value of plan assets 2,171 2,338 2,810
US | Pensions | Unquoted | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 339 401 403
US | Pensions | Unquoted | Government securities      
Disclosure of fair value of plan assets [line items]      
Debt instruments 460 467 444
US | Other post-retirement benefits      
Disclosure of fair value of plan assets [line items]      
Equities 589 553 561
Diversified alternatives 121 103 101
Other 187 163 161
Fair value of plan assets 2,731 2,658 2,631
US | Other post-retirement benefits | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 1,378 1,397 1,397
US | Other post-retirement benefits | Government securities      
Disclosure of fair value of plan assets [line items]      
Debt instruments 456 442 411
US | Other post-retirement benefits | Quoted      
Disclosure of fair value of plan assets [line items]      
Equities 36 31 37
Diversified alternatives 121 103 92
Other 2 0 0
Fair value of plan assets 1,831 1,925 1,890
US | Other post-retirement benefits | Quoted | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 1,217 1,350 1,351
US | Other post-retirement benefits | Quoted | Government securities      
Disclosure of fair value of plan assets [line items]      
Debt instruments 455 441 410
US | Other post-retirement benefits | Unquoted      
Disclosure of fair value of plan assets [line items]      
Equities 553 522 524
Diversified alternatives 0 0 9
Other 185 163 161
Fair value of plan assets 900 733 741
US | Other post-retirement benefits | Unquoted | Corporate bonds      
Disclosure of fair value of plan assets [line items]      
Debt instruments 161 47 46
US | Other post-retirement benefits | Unquoted | Government securities      
Disclosure of fair value of plan assets [line items]      
Debt instruments £ 1 £ 1 £ 1