Prescribed medical cannabis doesn’t exempt you from the 2µg/L THC roadside...

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

Prescribed medical cannabis doesn’t exempt you from the 2µg/L THC roadside limit. A statutory defence exists, but it won’t protect you if impaired. You must meet DVLA fitness-to-drive standards—a prescription isn't a safety substitute