The file you're looking for is /map/europe.mbd in base.scs. This is also easy to mod with a hex editor. The definition of that is probably hardcoded. I think the only correction is that rural UK is 1:15, probably based on country code. There is a screen somewhere allowing you to set urban (1:3) and rural (1:19) world scale, as well as whether corrections are used. This happens to be a thing you can do in the map editor.
