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

https://foxtrot-wiki.win/index.php/Lane_Keeping_and_Adaptive_Cruise:_Why_Your_Car_Isn%27t_Driving_You_Home

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.