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

https://bizzmarkblog.com/the-motorists-guide-to-medical-cannabis-dvla-rules-section-5a-and-staying-legal/

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