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

https://www.sierrabookmarking.win/prescribed-medical-cannabis-the-2-g-l-thc-limit-applies-to-everyone-while-a

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.