Pensions and other post-retirement benefits - Disclosure of amounts recognised in the statement of financial position (Details) - GBP (£) £ in Millions	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2022
Disclosure of defined benefit plans [line items]
Other post-employment liabilities	£ (52)	£ (69)
Net defined benefit asset	1,814	1,951	£ 3,075
Liabilities	(593)	(694)
Assets	2,407	2,645
Funded obligations
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(17,601)	(18,934)
Fair value of plan assets	19,733	21,246
Funding surplus/(deficit)	2,132	2,312
Unfunded obligations
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(266)	(292)
Pensions \| UK
Disclosure of defined benefit plans [line items]
Fair value of plan assets	11,782	12,578	16,865
Other post-employment liabilities	0	0
Net defined benefit asset	1,261	1,614	2,590
Liabilities	(56)	(58)
Assets	1,317	1,672
Pensions \| US
Disclosure of defined benefit plans [line items]
Fair value of plan assets	5,320	6,060	7,263
Other post-employment liabilities	0	0
Net defined benefit asset	408	324	484
Liabilities	(210)	(234)
Assets	618	558
Pensions \| Funded obligations \| UK
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(10,465)	(10,906)
Fair value of plan assets	11,782	12,578
Funding surplus/(deficit)	1,317	1,672
Pensions \| Funded obligations \| US
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(4,702)	(5,502)
Fair value of plan assets	5,320	6,060
Funding surplus/(deficit)	618	558
Pensions \| Unfunded obligations \| UK
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(56)	(58)
Pensions \| Unfunded obligations \| US
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(210)	(234)
Other post-retirement benefits \| US
Disclosure of defined benefit plans [line items]
Fair value of plan assets	2,631	2,608	2,885
Other post-employment liabilities	(52)	(69)
Net defined benefit asset	145	13	£ 1
Liabilities	(327)	(402)
Assets	472	415
Other post-retirement benefits \| Funded obligations \| US
Disclosure of defined benefit plans [line items]
Present value of funded obligations	(2,434)	(2,526)
Fair value of plan assets	2,631	2,608
Funding surplus/(deficit)	197	82
Other post-retirement benefits \| Unfunded obligations \| US
Disclosure of defined benefit plans [line items]
Present value of funded obligations	£ 0	£ 0

The present value, without deducting any plan assets, of expected future payments required to settle the obligation resulting from employee service in the current and prior periods. [Refer: Plan assets, at fair value]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
-Paragraph 57
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2023-en-r&anchor=para_57_a&doctype=Standard
-URIDate 2023-03-23

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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.

+ References

No definition available.

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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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 19
-IssueDate 2023-01-01
-Paragraph 140
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=19&code=ifrs-tx-2023-en-r&anchor=para_140_a&doctype=Standard
-URIDate 2023-03-23

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- Definition

The fair value of defined benefit plan assets. Plan assets comprise assets held by a long-term employee benefit fund and qualifying insurance policies. [Refer: At fair value [member]]

+ References

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- Definition

The amount of 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/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2023-01-01
-Paragraph 55
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2023-en-r&anchor=para_55&doctype=Standard
-URIDate 2023-03-23

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- Definition

The amount of deficit in a defined benefit plan. [Refer: Defined benefit plans [member]]

+ References

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- Definition

The fair value of any plan assets, less the present value of the defined benefit obligation. [Refer: Plan assets [member]]

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- Definition

Other Post-Employment Liabilities

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