Financial instruments - Market risk - Additional information (Details) - Market risk - GBP (£) £ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Disclosure of detailed information about financial instruments
Percentage of borrowings at fixed rates	57.00%
Reasonably possible reduction in short-term interest rates	1.00%
Decrease in net finance costs due to reasonably possible reduction in short-term interest rates	£ 26	£ 25
Reasonably possible increase in short-term interest rates	1.00%
Increase in net finance costs due to reasonably possible increase in short-term interest rates	£ 26	25
Decrease in equity due to reasonably possible decrease in short-term interest rate.	0	0
Increase in equity due to reasonably possible increase in short-term interest rate.	£ 0	0
Highly predictable future cash flows hedging period	24 months
Assumed weakening of foreign currencies	10.00%
Decrease in carrying value of net assets, excluding net borrowings, due to assumed weakening of foreign currencies	£ 835	892
Decrease in net borrowings, due to assumed weakening of foreign currencies	£ 716	671
Assumed strengthening of foreign currencies	10.00%
Increase in carrying value of net assets, excluding net borrowings, due to assumed strengthening of foreign currencies	£ 835	892
Increase in net borrowings, due to assumed strengthening of foreign currencies	716	671
Decrease in net profit, due to assumed weakening of foreign currencies	145	126
Increase in net profit, due to assumed strengthening of foreign currencies	145	£ 126
US dollar LIBOR
Disclosure of detailed information about financial instruments
Bonds swapped	£ 1,112
Weighted average maturity period of swapped bonds	4 years
Euribor
Disclosure of detailed information about financial instruments
Bonds swapped	£ 1,083
Weighted average maturity period of swapped bonds	4 years 7 months 6 days
Scientific, technical & medical | Subscriptions
Disclosure of detailed information about financial instruments
Highly predictable future cash flows hedging period	50 months