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

https://star-wiki.win/index.php/What_Does_the_Medical_Defence_Actually_Cover_for_Cannabis_Patients%3F_A_Guide_for_Drivers

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