From what you've said (and correct me if I've misunderstood), you've had this complaint a relatively short period of time. You/the physio thinks it's muscular, possibly a repeat of an old injury. You've not mentionned any trauma, other symptoms or that you have any weakness etc associated with this pain.
Is the Physio working?
To be honest a scan at this point may not really add to anything. If you've slipped a disk, in the absence of any neurological symptoms associated and with often effective non-operative treatment (Physio initially), I doubt you'd find a surgeon who would advise an operation at this point. And why would you want one before trying the non-surgical options first?
My point is....... given the relatively short course of the problem so far, stick with the physio....... if it's not working (bearing in mind muscular back pain can take weeks to recover) then see your doc and discuss a scan plus further treatment options. You may not need to worry about the medical records/access part of your post.
The part about access to notes - are you a pilot? Do you hold a medical already? If so which one? If you have a JAR medical, yes the CAA can request notes form your GP (I think you sign to approve this on the medical form). I don't work down there so whether they actually do or not I'm not sure.
Best of luck
B