Your symptoms sound to be severe but believe it or not, better for a potential solution. Your back pain as described seems to be from a typical severe degenerative disc disease condition called isolated disc resorption (IDR-see section under “causes of lower back pain” under “conditions”).
Your leg pain I was worried was caused by chronic radiculopathy (nerve injury that won’t heal-see website) but leg pain that increases with standing and walking also has a stenotic cause called foraminal or lateral recess stenosis. See the website for these conditions.
Your bowel condition may have nothing to do with your spine but I have seen patients with severe lower back pain have bowel control issues on occasion due to the intense nature of this lower back pain.
Maybe if you can make a case to the NHS for your back pain being caused by IDR and foraminal stenosis (possibly by using the information off this website and the paper in spine I just published which covers your condition), the NHS will consider a TLIF fusion of L4-S1 which very well could be the answer to your symptoms.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.