Navigating medical cannabis while driving is a minefield. While a medical...

https://wiki-view.win/index.php/What_Happens_If_I_Crash_and_THC_is_Found_in_My_Blood%3F

Navigating medical cannabis while driving is a minefield. While a medical defence exists, the 2mcg/L THC limit is strict. You must notify the DVLA of your fitness. Remember: a prescription doesn't waive road responsibility. Legality isn't impairment.