Medical cannabis is legal, but driving is a separate issue. Stick to the 2µg/L...

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

Medical cannabis is legal, but driving is a separate issue. Stick to the 2µg/L limit and inform the DVLA of your status. A medical defence isn’t a get-out-of-jail-free card; if you’re impaired, you’re liable. Legality never replaces responsibility.