Retirement Benefit Schemes - Schedule of Movements in Scheme Liabilities (Details) - Liabilities - GBP (£) £ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023
Changes in net defined benefit liability (asset) [abstract]
Present value beginning balance	£ 7,202	£ 7,312
Differences on exchange	(122)	(187)
Current service cost	38	37
Past service (credit)/cost and settlements	(221)	(66)
Interest on scheme liabilities	316	347
Contributions by scheme members	2	2
Benefits paid	(524)	(536)
Actuarial losses/(gains) arising from changes in demographic assumptions	(13)	(28)
Actuarial losses/(gains)arising from changes in financial assumptions	(245)	277
Experience losses/(gains)	6	44
Present value ending balance	6,439	7,202
Pension schemes
Changes in net defined benefit liability (asset) [abstract]
Present value beginning balance	6,647	6,697
Differences on exchange	(127)	(153)
Current service cost	37	36
Past service (credit)/cost and settlements	(221)	(67)
Interest on scheme liabilities	288	315
Contributions by scheme members	2	2
Benefits paid	(470)	(484)
Actuarial losses/(gains) arising from changes in demographic assumptions	(13)	(28)
Actuarial losses/(gains)arising from changes in financial assumptions	(239)	268
Experience losses/(gains)	14	61
Present value ending balance	5,918	6,647
Healthcare schemes
Changes in net defined benefit liability (asset) [abstract]
Present value beginning balance	555	615
Differences on exchange	5	(34)
Current service cost	1	1
Past service (credit)/cost and settlements	0	1
Interest on scheme liabilities	28	32
Contributions by scheme members	0	0
Benefits paid	(54)	(52)
Actuarial losses/(gains) arising from changes in demographic assumptions	0	0
Actuarial losses/(gains)arising from changes in financial assumptions	(6)	9
Experience losses/(gains)	(8)	(17)
Present value ending balance	£ 521	£ 555

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in demographic assumptions that result in remeasurements of the net defined benefit liability (asset). Demographic assumptions deal with matters such as: (a) mortality; (b) rates of employee turnover, disability and early retirement; (c) the proportion of plan members with dependants who will be eligible for benefits; (d) the proportion of plan members who will select each form of payment option available under the plan terms; and (e) claim rates under medical plans. [Refer: Net defined benefit liability (asset)]

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- Definition

The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from changes in financial assumptions that result in remeasurements of the net defined benefit liability (asset). Financial assumptions deal with items such as: (a) the discount rate; (b) benefit levels, excluding any cost of the benefits to be met by employees, and future salary; (c) in the case of medical benefits, future medical costs, including claim handling costs (ie the costs that will be incurred in processing and resolving claims, including legal and adjuster's fees); and (d) taxes payable by the plan on contributions relating to service before the reporting date or on benefits resulting from that service. [Refer: Net defined benefit liability (asset)]

+ References

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The decrease (increase) in a net defined benefit liability (asset) resulting from actuarial gains (losses) arising from experience adjustments that result in remeasurements of the net defined benefit liability (asset). Experience adjustments deal with the effects of differences between the previous actuarial assumptions and what has actually occurred. [Refer: Net defined benefit liability (asset)]

+ References

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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 [domain]]

+ 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 employee service in the current period. [Refer: Net defined benefit liability (asset)]

+ References

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

+ References

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-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

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-Name IAS
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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 [domain]]

+ References

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The increase (decrease) in the net defined benefit liability (asset) resulting from past service cost and losses (gains) arising from settlements. [Refer: Decrease (increase) in net defined benefit liability (asset) resulting from gains (losses) arising from settlements; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

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