Prescribed cannabis isn't a free pass. A medical defence exists for the 2mcg/L...

https://www.kilobookmarks.win/using-medical-cannabis-a-prescription-offers-a-statutory-defence-but-it

Prescribed cannabis isn't a free pass. A medical defence exists for the 2mcg/L THC limit, but it’s complex. Your priority is DVLA fitness to drive. Legality isn't a substitute for responsibility; impairment is an offence