The website Hipkiss’ Scans of Old Maps has the 1920 London Geographical Institute’s The People’s Atlas scanned as fairly high quality images.
It covers the whole world, but with larger and more detailed maps for the British Empire/Commonwealth and Europe. It also includes details on transport infrastructure and routes, travel times and similar which might be of interest for those planning a tabletop game campaign or RPG in the interwar era, and a number of maps of World War One (the Great War) and it’s aftermath.
The same site has Bartholomews Pocket Atlas and Guide to London from 1922 available, and a huge variety of other stuff on his Maps page.
Even better, the site owner points out that all his maps are from before 1923, therefore as far as he can tell, they’re public domain (out of copyright, that is) so there are no restrictions on what we can do with them.
Well worth looking at; I’ll probably get the files at work and print a few out too!