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

https://dlf-ne.org/can-i-drive-in-the-uk-if-i-have-a-medical-cannabis-prescription-the-reality-behind-the-wheel/

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