Legally prescribed cannabis is a reality, but road safety rules remain firm....

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

Legally prescribed cannabis is a reality, but road safety rules remain firm. Even with a valid medical defence, you must stay under the 2µg/L THC limit and satisfy DVLA fitness-to-drive requirements