Do many people pass their first medical and get denied entry at Raleigh? I visited my gp 3 years ago with back pain, he couldn't find a problem and an x-ray showed up nothing. I do get back ache when i'm doing physical labour but it doesn't stop me from working.
X-rays are reqested by GPs all the time for back pain - completely un-necessary. The only thing they show up is bones and unless you have had significant trauma they will be normal. There are few other plain x-rays which use more radiation than a lumbar spine x-ray. I'd refuse to have one if I was offered it.
Back pain is a dodgy one. Service life puts significant strain on your back, and if it's not 100% before you start then it's sort of asking for it. If you've had more than one episode then things may be awkward - the AFCO ME will do a careful assessment but it sounds like your back causes a bit of bother and you may run into difficulties.