Military flights in civil airspace should follow civil airspace rules wrt altimetry.
What is 'civil airspace'? I'm not familiar with the term in the UK.
Anyway, aircraft flying in class G in UK (civil or mil) may fly on any pressure they please, as long as they remain clear of controlled airspace, etc...
The example given is a situation that occurs very often in the UK. A military aircraft may be flying on RPS well above the TA and need to be co-ordinated against another aircraft (receiving a RAS) flying SAS. Some pressure based arithmetic needs to be done to ensure standard separation is maintained.
The reason a (typically military) aircraft might be flying on RPS is that it may be operating in a large altitude block, but requires to be accurately aware of it's base altitude to guarantee the flight isn't conducted dangerously low. This may even be against a Minimum Sector Safety Altitude which is greater than the Transition Altitude.
A considerate controller working such an aircraft should be aware of the equivalent airspace block being used in terms of Flight Levels and so a request to some of our civilian colleagues to coordinate not above, say, 22,500ft on an RPS of 1038 against traffic at FL 230 should not be necessary. The controller will know that 22,500ft on an RPS of 1038 equates to 21,750ft on SAS and ought to therefore offer co-ordination 'not above FL220'.