Prescribed cannabis isn’t a free pass. The 2mcg/L THC limit applies regardless...

https://noon-wiki.win/index.php/How_do_police_test_for_THC_in_the_UK%3F_A_guide_to_Section_5A

Prescribed cannabis isn’t a free pass. The 2mcg/L THC limit applies regardless of your prescription. You must notify the DVLA, and while a medical defence exists, legality doesn’t replace responsibility