v3.26.1
Pensions and other post-retirement benefits - Schedule of reconciliation of changes in fair value of plan assets (Details) - GBP (£)
£ in Millions
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset £ 1,916  
Changes in net defined benefit liability (asset) [abstract]    
Employer contributions 120 £ 282
Closing net defined benefit asset 2,147 1,916
Actual return on plan assets 982 (207)
Expected contributions to plans in the following year 93 108
Aviva    
Changes in net defined benefit liability (asset) [abstract]    
Actuarial gains (losses) arising from changes in demographic assumptions, net of tax, defined benefit plans 60  
UK | Pensions    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 1,128  
Changes in net defined benefit liability (asset) [abstract]    
Employer contributions 93 112
Closing net defined benefit asset 1,071 1,128
Actual return on plan assets 481 (530)
Expected contributions to plans in the following year 55 89
US | Pensions    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 476  
Changes in net defined benefit liability (asset) [abstract]    
Employer contributions 19 27
Closing net defined benefit asset 413 476
Actual return on plan assets 266 241
Expected contributions to plans in the following year 28 19
US | Other post-retirement benefits    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 312  
Changes in net defined benefit liability (asset) [abstract]    
Employer contributions 8 143
Closing net defined benefit asset 663 312
Actual return on plan assets 235 82
Expected contributions to plans in the following year 10 0
Plan assets    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 18,441 19,733
Changes in net defined benefit liability (asset) [abstract]    
Interest income 910 997
Return on plan assets (less than)/in excess of interest 72 (1,204)
Administration costs (27) (22)
Assets distributed on settlements (443) 0
Employer contributions 120 282
Employee contributions 5 5
Benefits paid (1,107) (1,168)
Exchange adjustments (191) (182)
Closing net defined benefit asset 17,780 18,441
Plan assets | UK | Pensions    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 10,603 11,782
Changes in net defined benefit liability (asset) [abstract]    
Interest income 510 601
Return on plan assets (less than)/in excess of interest (29) (1,131)
Administration costs (19) (14)
Assets distributed on settlements 0 0
Employer contributions 93 112
Employee contributions 5 5
Benefits paid (722) (752)
Exchange adjustments 0 0
Closing net defined benefit asset 10,441 10,603
Plan assets | US | Pensions    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 5,180 5,320
Changes in net defined benefit liability (asset) [abstract]    
Interest income 263 265
Return on plan assets (less than)/in excess of interest 3 (24)
Administration costs (6) (6)
Assets distributed on settlements (443) 0
Employer contributions 19 27
Employee contributions 0 0
Benefits paid (280) (282)
Exchange adjustments (128) (120)
Closing net defined benefit asset 4,608 5,180
Plan assets | US | Other post-retirement benefits    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset 2,658 2,631
Changes in net defined benefit liability (asset) [abstract]    
Interest income 137 131
Return on plan assets (less than)/in excess of interest 98 (49)
Administration costs (2) (2)
Assets distributed on settlements 0 0
Employer contributions 8 143
Employee contributions 0 0
Benefits paid (105) (134)
Exchange adjustments (63) (62)
Closing net defined benefit asset 2,731 2,658
Present value of defined benefit obligation    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset (16,401) (17,867)
Changes in net defined benefit liability (asset) [abstract]    
Interest income (785) (899)
Employee contributions (5) (5)
Benefits paid 1,147 1,203
Exchange adjustments 178 162
Closing net defined benefit asset (15,345) (16,401)
Present value of defined benefit obligation | UK | Pensions    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset (9,475) (10,521)
Changes in net defined benefit liability (asset) [abstract]    
Interest income (452) (533)
Employee contributions (5) (5)
Benefits paid 725 756
Exchange adjustments 0 0
Closing net defined benefit asset (9,370) (9,475)
Present value of defined benefit obligation | US | Pensions    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset (4,704) (4,912)
Changes in net defined benefit liability (asset) [abstract]    
Interest income (241) (246)
Employee contributions 0 0
Benefits paid 280 282
Exchange adjustments 121 110
Closing net defined benefit asset (4,195) (4,704)
Present value of defined benefit obligation | US | Other post-retirement benefits    
Disclosure of net defined benefit liability (asset) [line items]    
Opening net defined benefit asset (2,222) (2,434)
Changes in net defined benefit liability (asset) [abstract]    
Interest income (92) (120)
Employee contributions 0 0
Benefits paid 142 165
Exchange adjustments 57 52
Closing net defined benefit asset £ (1,780) £ (2,222)