Is it possible for the developers to provide a map larger than they anticipated, say way North of London with correct terrain elevations and airfields of either grass or concrete with enough spawn points on each and then let the players who are mission makers populate each airfield as they need it for their missions from objects within the game? There are plenty of photos of of airfields from that period, especially in England, which show the orientation of the runways and the extent of the surrounding buildings constructed to support the men and planes.
If the developers provide the skeleton of these airfields/strips, then the community can populate them as necessary.
In this way the general map would already be approved by RGB and the developers but they wouldn't need to spend the very large amount of time putting in the essential details of every field, town and village. Even London itself could be "skeletonized" and the community could add what details they desire.
We did this in IL2 and it worked out fine as long as the developers provide us with enough of objects (building and structure types) necessary for populating areas.