<<In the UK, the transition Altitude is the highest Altitude at which QNH is used as the controlling reference.>>
There's a bit more:
Within the UK:
Transition Altitude
Transition altitude is the altitude at or below which the vertical position of an aircraft is normally controlled by reference to altitude. Wherever possible there is a common transition altitude for aerodromes within a control zone (Eg London TMA it is 6000 ft). Unless otherwise notified the transition altitude for civil aerodromes is 3000 feet.
Transition Level
Transition level is the lowest flight level available for use above the transition altitude.
It is determined by ATC as follows:
a) Within controlled airspace by the controlling authority and it will normally be based on the QNH of the major aerodrome;
b) Outside controlled airspace by the aerodrome authority and based on the
aerodrome QNH.