UK AIP ENR 1.7 Altimeter Setting Procedures:
5.1.4 Within Controlled Airspace a pilot should set one altimeter to the latest Aerodrome QNH prior to take-off. 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.
5.1.5 Outside Controlled Airspace, a pilot may use any desired setting for take-off and climb. However, when under IFR, vertical
position must be expressed in terms of Flight Level on climbing through the Transition Altitude.
This is based on ICAO PANS-OPS (Doc 8168) Vol I, Part III (Aircraft Operating Procedures), Sec 1 (Altimeter Setting Procedures), Ch 1:
1.1(c) The change in reference from altitude to flight levels, and vice versa, is made:
1) at the transition altitude, when climbing; and
2) at the transition level, when descending.
The transition altitude is published. The transition level is determined by the aerodrome QNH. The altimeter subscale setting for flight levels is strictly 1013.2 hPa and not 1013 or 1013.25 hPa.
In the US and Canada the standard altimeter setting is set on the Standard Pressure Region side (which can be entered in level flight).