As Heathrow is in the UK, the following from the UK AIP ENR would apply:
GENERAL
Vertical positioning of aircraft when at, or below, any Transition Altitude will normally be expressed in terms of Altitude.
Vertical positioning at, or above, any Transition Level will normally be expressed in terms of Flight Level.
When descending through the Transition Layer vertical position will be expressed in terms of Altitude, and when climbing in terms of Flight Level. It should not be assumed that separation exists between the Transition Altitude and the Transition Level.
CLIMB
While flying at, or below, the Transition Altitude vertical position will be expressed in terms of altitude based upon the Aerodrome QNH. 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.
DESCENT
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.
Other States may do it differently, so check their AIPs before flight in their airspace.