Prescribed cannabis isn't an immunity pass. You must stay under the 2mcg/l THC...

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

Prescribed cannabis isn't an immunity pass. You must stay under the 2mcg/l THC limit and satisfy DVLA fitness requirements. A medical defence exists, but legality doesn’t replace your responsibility to drive safely. Don't let a prescription catch you out.