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

https://www.blaze-bookmarks.win/ev-drivers-using-medical-cannabis-legality-isn-t-immunity-a-2-g-l-thc-limit

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.