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

https://rapid-wiki.win/index.php/Is_there_an_official_DVLA_page_for_prescribed_cannabis_and_driving%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.