I agree with MrsD about the past antibiotics or recent treatments possibly setting this into motion.

I would certainly go forward with the skin biopsy now that other tests are normal and they are actually recommending it (which you indicated could be tough to get done in the NHS). The nerve conduction studies CANNOT determine small fiber neuropathy, so no surprise they are normal. The skin biopsy, will confirm or rule out the SFN, as not only can it determine the density of fibers, but also look at condition of fibers for any abnormalities. It won't tell the you cause of the neuropathy, but it will confirm it's presence and give you a baseline for level of severity.