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

https://wiki-tonic.win/index.php/What_happens_if_I_crash_and_THC_is_found_in_my_blood%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.