Hi,
Please excuse me butting in here, I normally mainly post on the aircraft forum but feel I may have something to add to this.
Whilst it is true that at the start of the war most of the airfields with asphalt/concrete runways were bomber bases by the middle of 1942 Fighter command also had a number that benefited from asphalt. Examples being Kenley,Coltishall, Martlesham, Duxford. In my opinion it would be perfectly acceptable to have a 'Generic' diorama base depicting asphalt or concrete hardstanding with grass verges, all airfields have/had grass verges and concrete aplenty!