In reply to OneLifeOneHeart:
You won't get referred for an MRI unless you have any red flag symptoms related to severe nerve root compression or cord compression, such as
severe pain from sciatic nerve down into toe
muscle weakness/foot drop
saddle parasthesia (pins and needles)
chronic sciatica which does not respond to conservative treatment and interferes with day to day living (and that doesn't include being able to run/climb etc but is more related to being unable to walk, unable to dress, unable to drive, unable to work, unable to sit etc)
GP's are now able to refer for MRI of the lumbar spine but not for simple back pain which is generally muscular/mechanical in nature (unless there is clinical indication of compression fracture or spondylolisthesis where one vertebrae slips over another.)
GP's cannot refer for cervical spine MRI but they can refer for x-ray to check for signs of wear and tear (though it's pretty much a waste of time and an unneeded dose of radiation as it won't affect management plan).
Your GP's attitude is shocking however and I wouldn't tolerate being spoken to by them in that manner! Get a second opinion and make sure you get a referral for a Physio. They should be able to determine whether there is any risk of nerve involvement. However, unless surgery is an indication there is still no need for an MRI. Scans only tend to be given if there is clinical indication that one is required, resulting in a changing or decision making process of management.
Finally,
I've had surgery on my spine for disc problems
I've got arthritis in my neck which affects my ability to climb and to run
I've got arthritis in my knee which flares up from time to time
Just been diagnosed with arthritis in my foot which can hurt like buggery.
My main piece of advice is see a physio or other therapist that could help, actually follow the advice that they give you. Don't let pain, however chronic rule your life. Bloody well take control, find ways to keep on enjoying being active whether it's by necking painkillers or reducing your pace running, downgrading your climbing etc and simply man the f*ck up.