Medical cannabis users have a defence against the 2mcg/L THC limit, but...

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

Medical cannabis users have a defence against the 2mcg/L THC limit, but legality isn't immunity. You must stay fit to drive per DVLA rules. Being prescribed doesn't replace your duty to remain unimpaired. Responsibility remains yours, always.