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

https://www.bookmark-fuel.win/prescribed-medical-cannabis-is-increasingly-common-but-it-creates-a-legal

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