One slant is whether the uni city has a neighbouring city of interest. Bristol has Bath, which imo wasn't that interesting as I recall, nice for a day trip. Eg go to Manchester, and Liverpool is half an hour on the train, so over the course of three years you could get to know two major UK cities.
Is Durham that far away from Newcastle?
It's very hard sussing all this stuff out because as the OP suggests, you spend your time revising and don't have the money or time to visit these cities, and do you really get the correct vibe of it visiting as a sort of tourist, say when the students aren't even there? Plus there is the sense of tempting fate to do all this before your results, like counting chickens before the eggs are hatched.
But then again, even if you wait for the results and then apply, you don't have much time if I recall, applications have to go off in October, if you visit over the summer, the students won't be there. Plus if you have your gap year it might change your perspective on what you want - which tbf is almost the point of a gap year, to help you develop as a person.
I agree with the person who suggested Bristol had a lot of posh people ('West London') there, that was my experience. There wasn't any diversity, of course all that may well have changed now, but it wasn't something that would have concerned me at all at the time - until I went there. I was blinded by academic snobbery.