BV,
As I do not fly RAF Aircraft and have no idea what capability your altimeters have, and knowing the RAF is bound to have some policy, SOP, or Pilot Instructions re setting of Altimeters, perhaps you or some other RAF Pilot could answer that simple straight forward question.
It would be educational to have an explanation of how you determine when QFE settings will not be feasible and thus necessitate your use of QNH instead. That would also tell us what the limitations of the QFE method might be too.
Nothing sinister here, just somewhat curious how you go about your business as it is much different than the way most of us are experienced in doing.