@tz I use ECMWF. The heights that I gave previously were all QFE (AGL) at Lasham.

@tz Experience from today Sunday 30th May 2021 at Lasham airfield near Basingstoke UK. Your forecast at 12:00 UTC was 4900ft with a temperature of 17 degrees C, rising to 5300ft at 15:00 UTC. Actual at 12:00 UTC was 2000ft at a temperature of 17 degrees C. By 15:00 UTC the thermic height was 2500ft rising to 4000ft by 16:30 UTC. Trust this helps.

I am a glider pilot and feel that your thermic prediction look rather optimistic for the UK. Are you predicting the cloud tops rather than the useable thermic height?