They are actually "underspent" accounts. So, the department gets enough to do their job. Should they not spend it all by the end of the year (presumably April), that is called an 'underspend'. That is what they are taking it back. Same with Scotland and Wales share of the budget.
I think you will find lots of departments buying things they don't really need or don't need straight away, next March.
Lack of public toilets in towns.
To think that London, or anywhere else for that matter, does not belong to any one demographic