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

https://augustjygg178.almoheet-travel.com/if-i-am-over-2-micrograms-of-thc-will-i-be-arrested-even-with-a-prescription

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.