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

https://dlf-ne.org/can-i-drive-in-the-uk-if-i-have-a-medical-cannabis-prescription-the-reality-behind-the-wheel/

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.