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

https://station-wiki.win/index.php/Driving,_Cannabis,_and_the_Law:_A_Practical_Guide_for_Patients

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