Pensions and other post-retirement benefits - Schedule of reconciliation of the net defined benefit asset (liability) (Details) - GBP (£) £ in Millions	12 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	£ 1,951	£ 3,075
Changes in net defined benefit liability (asset) [abstract]
Income/(cost) recognised in the income statement (including discontinued operations)	(44)	(91)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	(224)	(1,323)
Employer contributions	165	284
Other movements	(17)	(24)
Reclassification to held for sale (note 10)	(17)	30
Closing net defined benefit asset	1,814	1,951
UK \| Pensions
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	1,614	2,590
Changes in net defined benefit liability (asset) [abstract]
Income/(cost) recognised in the income statement (including discontinued operations)	17	(22)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	(474)	(1,183)
Employer contributions	118	197
Other movements	3	2
Reclassification to held for sale (note 10)	(17)	30
Closing net defined benefit asset	1,261	1,614
US \| Pensions
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	324	484
Changes in net defined benefit liability (asset) [abstract]
Income/(cost) recognised in the income statement (including discontinued operations)	(36)	(30)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	94	(206)
Employer contributions	26	76
Other movements	0	0
Reclassification to held for sale (note 10)	0	0
Closing net defined benefit asset	408	324
US \| Other post-retirement benefits
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	13	1
Changes in net defined benefit liability (asset) [abstract]
Income/(cost) recognised in the income statement (including discontinued operations)	(25)	(39)
Remeasurement and foreign exchange effects recognised in the statement of other comprehensive income	156	66
Employer contributions	21	11
Other movements	(20)	(26)
Reclassification to held for sale (note 10)	0	0
Closing net defined benefit asset	£ 145	£ 13

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- Definition

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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No definition available.

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The decrease (increase) in a net defined benefit liability (asset) resulting from the remeasurement of that net defined benefit liability (asset). [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The increase (decrease) in the net defined benefit liability (asset) resulting from expense (income) included in profit or loss. [Refer: Net defined benefit liability (asset); Profit (loss)] [Contrast: Post-employment benefit expense in profit or loss, defined benefit plans]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from changes that the entity does not separately disclose in the same statement or note. [Refer: Net defined benefit liability (asset)]

+ References

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The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

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Increase (Decrease) In Net Defined Benefit Liability (Asset) Through Classification As Held For Sale

+ References

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