I had to cancel my hols due to the company and, after a bit of negotiation, the insurers look like they're going to pay up. The expression they used was "...if as a member of HM Forces, Police etc you are unexpectedly and unavoidably called upon for emergency duty, with a supporting letter from your supervisor etc..." Providing you have your boss include the 3 key words - unexpectedly, unavoidably and emergency, they can't really argue. (mine was, btw, so it wasn't fraud !!)
As for the RAF, they have a little-publicised form to claim "nugatory holiday expenses" which will (or should) reimburse you up to £200 of qualifying costs. This also includes the actual premium you paid (makes sense - why should you be out of pocket buying insurance for a holiday you couldn't go on ?), but 'an adminner' says that although it says so fairly clearly on the form, they don't tend to pay that bit. As ever, be nice to the adminners and you might be lucky !