@Bee Eater
My father had similar experience in the UK with the NHS. It's certainly not unusual for public health systems to apply some kind of age criterion when the "cost-benefit" of expensive ops might be debatable. I don't think these things are set in stone, so it's likely that the squeaky wheel will get the oil. :-) I think you can try again with your GP, and explain that you're really in a lot of pain and want to be referred for x-rays and a consultant. It probably won't hurt to point out that you have an S1, so your treatment would actually be paid for by the UK. (Also it's not clear from your post whether the advice came from your GP directly, or was broad guidance from your medical centre receptionist. In which case, get an appointment with your GP and see what he says when you discuss it directly with him.)
Alternatively, have a (private) consultation with another doctor (or specialist orthopedist), and tell them your problem and ask for their advice on how to navigate the Spanish system, given the feedback from your GP.
I don't know what connections you still have with the UK, but perhaps there's a way to get the NHS to do something for you.
If you want to go private, it's usually very easy, and you can have pretty much whatever treatment you want, when you want, regardless of your age. But it's typically very expensive, and costs vary widely. Hip and knee replacements are very popular ops, so there are plenty of options in the 10-15k euros range, and maybe a few coming in just below 10k. And lots who will charge 20k, 30k, and up (especially in the USA).
Usually, you start off by picking a private hospital (by recommendation/reviews/cost/convenience) and paying for the appropriate scan (e.g. x-rays of your knee) and a consultation/review with the appropriate specialist (e.g. orthopedic surgeon). A lot of folks will do ops internationally to save money. Turkey, in particular, is a very popular spot for European health tourism as it's very cheap and quite quick/easy to fly to Istanbul. For Brits, going back to the UK is also a very popular option as there are lots of very high quality private hospitals, no language issues, and the cost is not outrageous (maybe 15-20k euros for hips/knees).
Many of the international options specialize and behave like "focused factories" doing lots of hip replacements, or dental veneers, or boob jobs, or whatever. In my own hip research I spotted Nordorthopaedics who have good pricing (sub-10) and excellent reviews, and might be worth investigating. Dr. Google will find you a ton of options when you search for "hip replacement" or "knee replacement".