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

https://dlf-ne.org/can-i-drive-in-the-uk-if-i-have-a-medical-cannabis-prescription-the-reality-behind-the-wheel/

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