Medical cannabis is legal, but the 2mcg/L THC limit is a strict roadside...

https://erickcmpc682.theglensecret.com/i-feel-fine-how-do-i-know-if-i-am-actually-safe-to-drive

Medical cannabis is legal, but the 2mcg/L THC limit is a strict roadside reality. While a medical defence exists, it isn't immunity; ensure the DVLA clears your fitness. Legality is no substitute for responsibility: if impaired, don't drive.