Prescription cannabis doesn’t grant immunity from the 2mcg/L THC limit or DVLA...

https://evpowered.co.uk/feature/medical-cannabis-and-uk-driving-laws-what-ev-owners-must-know/

Prescription cannabis doesn’t grant immunity from the 2mcg/L THC limit or DVLA standards. A medical defence exists, but it’s no safety pass. Legality doesn’t replace your duty to drive unimpaired