If you’re prescribed medical cannabis, you must still adhere to the 2...

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

If you’re prescribed medical cannabis, you must still adhere to the 2 micrograms per litre THC limit. While a ‘medical defence’ exists, it isn't an automatic pass. You’re still required to report your condition to the DVLA to ensure you’re fit to drive