Medical cannabis is legal, but the 2mcg/L THC limit is a strict roadside...

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

Medical cannabis is legal, but the 2mcg/L THC limit is a strict roadside reality. While a medical defence exists, it isn't immunity; ensure the DVLA clears your fitness. Legality is no substitute for responsibility: if impaired, don't drive.