Hi Rod.
As a Brit living in London (and often travelling around these isles) I feel I should be able to help.
I suppose the first question is: what story do you plan to tell with this diorama?
Is it a city, town or rural scene (I guess from your suggested building that it's one if the first two).
Do you want to show Blitz damage? Are you going for the 'stiff upper lip'?
I would say that many buildings can be made to look 'British 1940s' if you go for the right details: e.g. windows cris-crossed with tape, the appropriate signage, red post boxes, etc...
Let us know more and I'm sure we can help!
Regards,
Tim