Well, nowadays there are a lot of airfields in Europe that won't let you run an APU on the ground for more than 15 minutes before departure (Stansted was one) and running an engine on the ramp was an absolute no-no.
But maybe you can negotiate with the Airport Operator/ATC to run engines and hold somewhere off the ramp?
Also generally there is an exception to APU usage rules when other methods of ground air etc are not available with higher OATs