Prescribed medical cannabis is legal, but driving is a separate issue. Even...

https://holdenajvs101.yousher.com/can-my-licence-be-invalidated-if-i-hide-medical-information-from-dvla

Prescribed medical cannabis is legal, but driving is a separate issue. Even with a prescription, exceeding the 2µg/L THC limit can lead to prosecution. While a medical defence exists, you must first satisfy DVLA fitness-to-drive requirements