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

https://wiki-tonic.win/index.php/What_happens_if_I_crash_and_THC_is_found_in_my_blood%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.