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

https://star-wiki.win/index.php/What_Does_the_Medical_Defence_Actually_Cover_for_Cannabis_Patients%3F_A_Guide_for_Drivers

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