I think the problem is that the letters stand for Imperial War Museum.
They do have civil aircraft in the exhibition hall, but it seems only those that have air force credentials.
Leaving aircraft outside does them no good at all. Down at the de Havilland Aircraft Museum those aircraft that are kept indoors remain presentable as exhibits, those left outside because there isn't enough room indoors suffer from rain, snow and ultra-violet radiation. Unless the hangar appeal drums up enough cash they will eventually end up as piles of aluminum oxide.