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

https://medium.com/@ada_chambers55/can-i-rely-on-the-medical-defence-if-i-was-driving-poorly-but-felt-fine-b45724e80299

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