What are the laws regarding marijuana in the UK?

I did a Google search, but I haven't found clear answers yet. Ideally, I would like medical marijuana so I don't have to worry about the legality of it, but I don't know what the process is in order to get it here. Do you have to be 18 or have an oncologist here prescribe it for you? I'm 17, and want it to help with nausea from chemo.