Sorry you're in UK. I hope they won't get so stingy with tests here, although I'm hearing that insurances here are refusing to pay after the test is done if it comes out negative.
I'm in the sciatica boat too, but at least got the MRI and because of that, treatment. I could walk well for a block, but the pain was bad. The MRI showed where the nerves were being impinged by the protruding discs. I've had a couple of years of injections in various spots and the discs may have at least partly healed themselves; at least the sciatic pain is much reduced. My pain doc was starting to ask me if I wanted surgery, but I wanted to keep trying alternatives a while longer. I'm glad I did, because he most recently injected tailbone (nerves to legs run through there), and it's lasted the longest and has worked the best. I want to postpone surgery as long as possible.
Ask the doc what's the cause of the sciatica. Mine is scoliosis plus injuries being in a bad wreck a couple of years ago.
I also have flat feet and arch supports with a medial wedge helps that. Because of the scoliosis, one leg is "functionally" shorter than the other, so I have a tiny lift on one orthotic. This helps the spine too!
I don't get much relief from ibuprofen. Acetominophen works as well, which isn't much at all. I can't take codeine, but Vicodin doesn't do much either. Tramadol, cheap generic, helps more - maybe because of the serotonin raising property. Some people get relief from SSRIs or SNRIs. I have fibromyalgia, hence the tramadol.