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

https://evpowered.co.uk/feature/medical-cannabis-and-uk-driving-laws-what-ev-owners-must-know/

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