Why so many UK cinemas closed in the mid-late 20th Century
As people began to buy televisions for their homes, they went out less. So cinemas had to adapt or go out of business. The major change was to split single cinemas up into smaller ones, so offering a larger range of programmes. However many cinemas simply closed.
Picture palaces/cinemas which were once (in the mid 20th Century) within easy travelling distance
The following cinemas were within easy reach of Edgware, my home town, when I was growing up in the 1940s and 50s.
Edgware: Ritz Definitely the classiest of them all as regards interior decor, decoration etc, and possibly also the largest; it also had The ABC Minors; and its organ was in regular use.
Burnt Oak: Gaumont and Savoy More or less the same as regards size, interior, etc. etc.
Colindale: Odeon Ditto regards size, interior, etc.
Hendon Central: Gaumont Ditto.
Old Hendon: Odeon Ditto.
Mill Hill: The 'Flea Pit' as we called it, which I think was called The Capitol. Much smaller and, as far as I remember it, did not have a balcony. You could be unlucky to have a 6" diameter cylindrical roof strut/column partially impairing your view!
I think there was also one in Hendon near Bell Lane, but I don't remember the name and I don't think it showed the latest films. It was presumably quite small.
All are gone. Anyone from Edgware wishing to go to a cinema today has to travel to Golders Green.
What now for the old cinema buildings
As an example, when the Edgware Ritz closed, it was gutted and turned into flats. Its shell was kept, as can be seen from the photographs. A new glassy front was added.