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

https://kilo-wiki.win/index.php/Presence_vs_impairment:_why_do_UK_drug_driving_rules_confuse_everyone%3F

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.