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

https://www.apu-bookmarks.win/if-you-re-an-ev-driver-prescribed-medical-cannabis-don-t-assume-legality

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