Having been involved in tasking AT assets in the UK and overseas I know that the decision to operate onto strips without CAA/FAA crash cover is not taken lightly.
Even if a task is in support of Ops whilst mounting out of a UK airfield, you can't use operational crash cover exemptions, unless you needed to move someone otherwise they would die (Aeromed?), but then I think the crew would also be on very thin ice if things went wrong.
I think that in the event of an accident where people died due to a lack of fire cover the coroner wouldn't be too pleased if the only reason that the cover wasn't in place was because the MOD didn't have enough money for fire cover at that specific airfield.
It all boils down to not enough money, crews, aircraft, equipment....the list goes on.