you can ask your GP to refer you to a specialist – the GP can say no but not likely to. I asked for a CT scan* and they said they can’t directly do this. I would have to wait till I see my consultant at the hospital in July but I’m getting referred for a colonoscopy at my request via the GP. The reason I asked for a scan is because the tests they have done have not shown anything up.
If your GP doesn’t agree that its worth you should ask why and get them to explain it in full details. If you are not satisfied with their reasons then go for a second opinion.
*I have an on going medical issue that hasn’t been diagnosed.