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

https://telegra.ph/Can-I-be-charged-if-I-am-legally-prescribed-but-the-officer-thinks-I-look-impaired-04-23

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.