If you’re prescribed medical cannabis, you must still adhere to the 2...

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

If you’re prescribed medical cannabis, you must still adhere to the 2 micrograms per litre THC limit. While a ‘medical defence’ exists, it isn't an automatic pass. You’re still required to report your condition to the DVLA to ensure you’re fit to drive