Medical cannabis is legal, but road safety rules apply. Keep under the 2µg/L...

https://dlf-ne.org/can-i-drive-in-the-uk-if-i-have-a-medical-cannabis-prescription-the-reality-behind-the-wheel/

Medical cannabis is legal, but road safety rules apply. Keep under the 2µg/L THC limit and notify the DVLA. A medical defence isn't a free pass; you must be fit to drive. Legality doesn't replace responsibility—ensure you’re safe behind the wheel.