It is for this reason that some (if any) aircraft may be permanently military registered to keep a minimum mil fleet available..
Rigga, I'm not doubting your knowledge on the ins and outs of aircraft registry - but surely there must be some way round this? I can't see how ATr can be run efficiently if some aircraft are for military use only. Likewise, it doesn't make sense (as others have suggested) to have RAF crew fly the AAR jobs, and airline crew fly the AT jobs. If ATr wants aircraft utilisation rates and aircrew rosta efficiences to match those of the airlines, then surely all aircrew and aircraft will need to be fully interchangeable.
That begs another question - will there be mixed crews of RAF and sponsored reservists? Will the sponsored reservists have RAF rank?