https://www.caa.co.uk/data-and-publications/publications/documents/content/dap-policy-106/
DAP Policy 106: Harmonisation of Transition Altitude in Controlled Airspace | UK Civil Aviation...
Summary information about publication DAP Policy 106
transition altitudecontrolled airspacecivil aviationdappolicy
https://www.flightcrewguide.com/guide/procedures-and-regulations/transition-altitude/
Transition Altitude - Flight Crew Guide
Transition Altitude explained: how states set it per aerodrome, rules for nearby airports, and the minimum 900m (3000ft) height.
transition altitudeflight crewguide