Prescribed cannabis users may have a medical defence against the 2µg/L THC...

https://city-wiki.win/index.php/Medical_Cannabis_and_Driving:_Your_DVLA_Responsibilities_and_the_Law

Prescribed cannabis users may have a medical defence against the 2µg/L THC limit, but it isn't a free pass. You must disclose your condition to the DVLA. Legal status never replaces the duty to drive safely. Impairment is impairment, prescription or not.