Prescribed medical cannabis isn't an immunity card. A medical defence exists...

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

Prescribed medical cannabis isn't an immunity card. A medical defence exists for the 2mcg/L THC limit, but DVLA fitness rules still apply. Legality doesn't replace responsibility; if you’re impaired, you shouldn't be driving. Stay safe and aware.