Hello scud - The position in the UK is that when an aircraft is descended from a Flight Level to an Altitude preparatory to commencing an 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 QNH will be set following the final Flight Level vacating report.
Similarly, when cleared to climb from an Altitude to a Flight Level vertical position is referred to in terms of Flight Level unless intermediate altitude reports have been requested by ATC.
The latter procedure was introduced a few years ago in an attempt to reduce the number of level busts caused when pilots forgot to change to the Standard Pressure Setting on
passing the Transition Altitude for reasons of workload, distraction etc. the trigger for changing to Standard Pressure now being the clearance to climb to a Flight Level.
In my sphere of operations which is one of the London Airports the Transition Level does not have much relevance so it is not routinely promulgated, but I cannot speak for other locations.
Hope this helps
