Hi. I just thought I'd offer what little bit of knowledge I have. I live in Canada and cannabis was just recently legalized. So up until October, there was a lot of confusion as to where and how to get CBD oil. First of all, in order for it to actually work (and not just be hemp oil, which is about as effective in treating pain as olive oil - but might offer other health benefits) it has to have a certain percentage of active ingredients in it which is also what made it illegal. There was no regulation, so all kinds of shops and online stores could sell CBD oil and it would really just be hemp oil, or something equally useless. 

Based on what I have read,  the best way to get it is through a doctor. I believe medical marijuana will be available in the U.K soon (if it isn't already) but like how it was here in Canada (and still is if you want medical marijuana), you have to go through a lot of paperwork, doctors, prescriptions etc. to get it. 

I wouldn't trust anywhere selling CBD oil. Not online, not in stores - unless you have stores that you can only access with a medical marijuana licence (or whatever it's called - through prescription).