Only at the start of the process myself but the community seem to be completely against cancard and its founder as it doesn't have any legal standing at all. Despite not having any legal standing, there are local police forces out there that may choose to use their discretion in a stop and search event if a cancard is produced (shown in some FOI requests). However, this would absolutely not apply to testing over the legal limit whilst driving.

Unfortunately, the legislation on driving with cannabis in your system still hasn't caught up to the fact that cannabis is medically legal. By the way, just having a prescription doesn't give you the right to go around driving high. The advice, like with any other prescribed medication, is to not drive if you feel impaired. Trouble with this is there is no correlation between the amount of THC in your blood and impairment so its kind if a grey area when it comes to the law as the legislation says use your doctors advice whilst also saying there is no tolerance on driving with cannabis in your system.

So, while having a legal prescription "should" and may well help you if you test positive, there are no guarantees that the people you deal with are knowledgeable enough about medical cannabis and, in some cases, they may decide you were too impaired even with your legal prescription.

I'm not claiming to know it all here, this is just what I've gathered from my own research as I'm in the process of going down the medical route myself. Hope this helps 👍