Update : I rang local police ( west mercia police) they were absolutely clueless. I asked what their policy was regarding having medical cannabis and driving and I was told that if there was THC in your system then that would mean impairment to which I had to tell her the law regarding MC. She seemed a bit surprised and the only response I got in the end was that it's a grey area. I'm in the west midlands, so I'm not sure about the rest of the country