From UK AIP ENR 1.7
"When cleared for climb to a Flight Level, vertical position will be expressed in terms of Flight Level, unless intermediate altitude reports have been
specifically requested by Air Traffic Control.
When an aircraft is descended from a Flight Level to an Altitude preparatory to commencing approach for landing, ATC will pass the appropriate aerodrome QNH. On vacating the Flight Level, the pilot will change to the aerodrome QNH unless further Flight Level vacating reports have been requested by ATC, in which case, the aerodrome QNH will be set following the final Flight Level vacating report."
In general therefore, unless ATC request otherwise, you can change to STD as soon as cleared climb to a flight level, and you can change to QNH as soon as you commence descent to a cleared altitude.
Exeptions are:
You must have QNH set for take off and can't change to STD below flap retraction altitude.
At least one altimeter (e.g. the standby) should remain on QHN below MSA for terrain awareness.