Prescribed cannabis is legal, but the 2mcg/L THC limit applies to all drivers....

https://www.insert-bookmark.win/driving-with-medical-cannabis-needs-more-than-paperwork-while-a-medical

Prescribed cannabis is legal, but the 2mcg/L THC limit applies to all drivers. While a prescription provides a statutory defence, it is not a free pass for impairment. You must still inform the DVLA of your fitness to drive