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.