>The UK census seems as good a measure as any.

Agreed, and it's a reasonable first reach for residential counts.

Problem is, it was last taken during 2021 - at the height of covid - and their estimates have been paying the price of it even since. Anything that touches on workday or employment they actively warn you away from, nowadays.

But then there's the elephant in the room: what if you're looking at somewhere like the City of London (the square mile one) where residential population is somewhere between 8,500 and 9,000 people?

EDIT: it slipped by me!

There is no single UK-wide census. NRS do Scotland's, last in 2022. NISRA handle Northern Ireland's census, last in 2021. And the ONS for England and Wales, last in 2021.