The purpose for which I asked my question is regarding counting, listing and visiting purposes. Retail purpose may - or may not - give the same answer.For National Rail retailing purposes, all the stations I mentioned are treated as a single entity.
My thoughts are tending towards saying if you enter a station, what you can get to without exiting that station (or requiring special authority to get to, e.g. by crossing tracks) is one station; if you have to exit, and then enter, you have entered a different station - whether that is a case of the exit from one being the entrance to the other, or requiring use or cossing over a public road (or othe non-railway land).
Where this perhaps does make an (unexpected) difference is those stations where there is no 'in station' route between up and down platforms, but you have to leave the station to do so.
On a couple of particular instances that have come up: on London Underground, I regard Bank and Monument as one (as TfL do, at least for some purposes).
And Paddington, as a whole, I think is 4: National Rail; TfL Elizabeth Line; LU Bakerlo/Circle/District (what was once referred to as Praed Street); and LU H&C (formerly Suburban/Bishops Road)