Prescribed cannabis is legal, but the 2µg/L THC limit applies to all. A medical...

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

Prescribed cannabis is legal, but the 2µg/L THC limit applies to all. A medical defence needs you to be unimpaired. You must tell the DVLA of your fitness. Legality isn't immunity; you remain responsible for safety