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

https://www.4shared.com/office/4hExEGiDku/pdf-94952-99400.html

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