Prescribed medical cannabis is legal, but for UK drivers, the 2μg/L THC limit...

https://wiki-cafe.win/index.php/What_does_%27taken_exactly_as_directed%27_mean_for_the_medical_defence%3F

Prescribed medical cannabis is legal, but for UK drivers, the 2μg/L THC limit remains. While a medical defence exists, it doesn’t override DVLA fitness-to-drive requirements