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

https://mag-wiki.win/index.php/I_feel_fine_%E2%80%93_how_do_I_know_if_I_am_actually_safe_to_drive%3F

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