Prescribed cannabis isn’t a free pass. The 2mcg/L THC limit applies regardless...

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

Prescribed cannabis isn’t a free pass. The 2mcg/L THC limit applies regardless of your prescription. You must notify the DVLA, and while a medical defence exists, legality doesn’t replace responsibility