Prescribed medical cannabis gives you a defence against the 2µg/L THC limit,...

https://www.save-bookmarks.win/driving-on-medical-cannabis-while-a-medical-defence-covers-the-2-micrograms

Prescribed medical cannabis gives you a defence against the 2µg/L THC limit, but it isn’t a blanket pass to drive impaired. As a motorist, you must still declare relevant conditions to the DVLA and ensure your ability to drive is unaffected