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

https://evpowered.co.uk/feature/medical-cannabis-and-uk-driving-laws-what-ev-owners-must-know/

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.