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

https://super-wiki.win/index.php/If_I_am_under_the_THC_limit,_am_I_definitely_safe_legally%3F

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.