Prescription cannabis doesn’t grant immunity from the 2mcg/L THC limit or DVLA...

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

Prescription cannabis doesn’t grant immunity from the 2mcg/L THC limit or DVLA standards. A medical defence exists, but it’s no safety pass. Legality doesn’t replace your duty to drive unimpaired