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, 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]
Employer contributions	165	284
Reclassification to held for sale (note 10)	(17)	30
Closing net defined benefit asset	1,814	1,951
Actual return on plan assets	302	(4,956)
Expected contributions to plans in the following year	151	149
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]
Employer contributions	118	197
Reclassification to held for sale (note 10)	(17)	30
Closing net defined benefit asset	1,261	1,614
Actual return on plan assets	(115)	(3,852)
Expected contributions to plans in the following year	108	99
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]
Employer contributions	26	76
Reclassification to held for sale (note 10)	0	0
Closing net defined benefit asset	408	324
Actual return on plan assets	202	(775)
Expected contributions to plans in the following year	28	36
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]
Employer contributions	21	11
Reclassification to held for sale (note 10)	0	0
Closing net defined benefit asset	145	13
Actual return on plan assets	215	(329)
Expected contributions to plans in the following year	15	14
Plan assets
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	21,246	27,013
Changes in net defined benefit liability (asset) [abstract]
Interest income	1,011	782
Return on plan assets (less than)/in excess of interest	(709)	(5,738)
Administration costs	(22)	(19)
Assets distributed on settlements	(513)	(571)
Employer contributions	165	284
Employee contributions	10	10
Benefits paid	(1,171)	(1,288)
Exchange adjustments	(195)	697
Reclassification to held for sale (note 10)	(89)	76
Closing net defined benefit asset	19,733	21,246
Plan assets \| UK \| Pensions
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	12,578	16,865
Changes in net defined benefit liability (asset) [abstract]
Interest income	620	398
Return on plan assets (less than)/in excess of interest	(735)	(4,250)
Administration costs	(13)	(9)
Assets distributed on settlements	0	0
Employer contributions	118	197
Employee contributions	10	10
Benefits paid	(707)	(709)
Exchange adjustments	0	0
Reclassification to held for sale (note 10)	(89)	76
Closing net defined benefit asset	11,782	12,578
Plan assets \| US \| Pensions
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	6,060	7,263
Changes in net defined benefit liability (asset) [abstract]
Interest income	271	273
Return on plan assets (less than)/in excess of interest	(69)	(1,048)
Administration costs	(7)	(8)
Assets distributed on settlements	(513)	(571)
Employer contributions	26	76
Employee contributions	0	0
Benefits paid	(312)	(426)
Exchange adjustments	(136)	501
Reclassification to held for sale (note 10)	0	0
Closing net defined benefit asset	5,320	6,060
Plan assets \| US \| Other post-retirement benefits
Disclosure of net defined benefit liability (asset) [line items]
Opening net defined benefit asset	2,608	2,885
Changes in net defined benefit liability (asset) [abstract]
Interest income	120	111
Return on plan assets (less than)/in excess of interest	95	(440)
Administration costs	(2)	(2)
Assets distributed on settlements	0	0
Employer contributions	21	11
Employee contributions	0	0
Benefits paid	(152)	(153)
Exchange adjustments	(59)	196
Reclassification to held for sale (note 10)	0	0
Closing net defined benefit asset	£ 2,631	£ 2,608

No definition available.

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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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- Definition

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

+ References

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- Definition

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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- Definition

The estimate of the expected contributions to be made to a defined benefit plan for the next annual reporting period. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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The increase (decrease) in the net defined benefit liability (asset) resulting from administration costs in the current period, that are not reflected in return on plan assets. [Refer: Administrative expenses; Net defined benefit liability (asset)] [Contrast: Administration costs not reflected in return on plan assets, defined benefit plans; Decrease (increase) in net defined benefit liability (asset) resulting from return on plan assets excluding interest income or expense]

+ References

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

The increase (decrease) in the net defined benefit liability (asset) resulting from changes in foreign exchange rates. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
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- Definition

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plan assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Increase (decrease) in net defined benefit liability (asset) resulting from interest expense (income)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Decrease (Increase) In Net Defined Benefit Liability (Asset) Resulting From Assets Distributed In Respect Of Settlements

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No definition available.

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- Definition

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

+ References

No definition available.

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Return On Plan Assets, Including Interest Income, Net Defined Benefit Liability (Asset)

+ References

No definition available.

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