Prescribed medical cannabis is legal, but driving is a separate issue. Even...

https://bizzmarkblog.com/the-motorists-guide-to-medical-cannabis-dvla-rules-section-5a-and-staying-legal/

Prescribed medical cannabis is legal, but driving is a separate issue. Even with a prescription, exceeding the 2µg/L THC limit can lead to prosecution. While a medical defence exists, you must first satisfy DVLA fitness-to-drive requirements