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

https://www.4shared.com/office/4hExEGiDku/pdf-94952-99400.html

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.