Medical cannabis users have a legal defence for the 2mcg/L THC limit, but...

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

Medical cannabis users have a legal defence for the 2mcg/L THC limit, but legality doesn’t equal road safety. DVLA fitness standards still apply. Having a prescription isn't a free pass; you remain responsible for your driving. Stay informed and safe.