This makes me absolutely livid, but I really don't know what can be done about that kind of treatment. I'll probably get shot down for saying this (by someone here who thinks the NHS is completely wonderful with no failings at all), but your story is not at all uncommon in the UK, and it is a real failing of the NHS. I've lived in another country with a national health system and it was streets ahead in terms of patient accountability and service (though still with problems of long waiting lists and budgets). NHS is great when you have an easily testable disorder, or one that can show up on a single round of basic tests. Its hopeless if you are trying to get to the bottom of a long term disorder, particularly one that you have put up with for years, or one that requires building up a clear picture of the clinical symptoms over a period of years.
Having said that though, you deserve a lot better treatment than you are getting. For a start you need to be seeing a rheumatologist that has a special interest in spondyloarthropathy (spondyloarthritis, or just generally inflammatory arthritis). Not all of them are up to date, and specially not with the way women can often present differently or much later. There also does seem to be a prevailing opinion that it is much better NOT to give someone a diagnosis (whether its neuro, or rheumatology or many other areas) than to say "suspected" or "probable" whatever. That is just so totally wrong because it cuts you off from getting treated or followed up appropriately.
I think if your GPs are generally supportive of a strong level of suspicion of AS, then you could definitely ask one of them to find out who the inflammatory arthritis specialists are (or you could ask who your relatives see and trust, or phone the advice line at NASS and get a name from them) and get referred directly to them. You are totally entitled to a second opinion in the NHS, but you need to make sure you are seen by the right person otherwise it may not get you any further. You could also look at talking with PALS (patient advice and liaison service at your local hospital) to see how they could help you get appropriate treatment or this second opinion. Its just not good enough that you are being dismissed and left like that.
Another thing to consider doing is to take someone with you to appointments - friend or family member, who can make sure that you remember to ask all the right questions, and who can take more note of what the doctor is saying. It sounds awful, but every time I have had someone else in with me, I find I get treated completely differently - far better, and the doctors go out of their way to listen to me and explain things properly and do adequate examinations.
I'm still shaking my head over how you have been treated. I can't get very far with treatment or diagnosis myself, but with your family history? wtf!!!!! Why can't they see it?????? What is going on here?????
The UK National Ankylosing Spondylitis Assocation website is here
http://www.nass.co.uk/ if you want to phone them and find out the names of rheumatologists with an interest in AS in your area (or anything else about AS).
