Trouble in Store
I was talking to a friend of mine yesterday and we talked about old films. He told me that he remembers as a boy being used as an extra, together with other school friends, in order to take part in interior shots for the 1953 Norman Wisdom film 'Trouble in Store.'
Although, as you say, the exterior shots were filmed in London, some of the interiors were shot inside Welwyn Department Store in Welwyn Garden City (now John Lewis).
Although he and his friends ended up on the cutting room floor some of the interiors of the old shop can be seen in the film.
I hope this information is of use. Incidentally, Welwyn Film Studios, now demolished, were also quite active from the early twenties and a number of well known films, including 'Brighton Rock', were made there.
Elstree film studios is also only a few miles up the A1.
..................thanks Colin, more useful info