You pm'd me. This is my 'open letter' reply.
You've already had some pretty good and helpful answers. The best I can add to them is:
1. I echo what Snoregone Conclusion said about surgery. It has risks, and you can't 'undo' it.
Best practice is to try everything else first.
2. If you're looking for help and treatment outside the NHS system – which you are – then you can buy your own machine.
And if it has to be a new one, then it's better, for several reasons, to buy direct from either of the two main manufacturers – Philips Respironics UK or ResMed UK.
And yes, under UK law, you do need a prescription – or 'scrip'. But that's not a big problem. If you call either PR UK (0800 130 0844) or ResMed UK 0800 907 7071) and ask for a scrip form, and they will send you a pdf version by e-mail.
You can either print that out and have your GP sign it. Or you can re-direct the pdf to your GP's surgery, and he or she can sign it electronically.
As to filling in the form, I've known one of these manufacturers accept a scrip without any pressure numbers being mentioned – the doctor's signature was enough. But if they insist, just put 'minimim = 4 cm; maximum= 20 cm'. In other words, the 'ex-works' or 'wide open' pressure settings.
3. You ask about servicing of a machine. Modern CPAP machines don't need much servicing. They are made to a very high standard. And the 'mean time to failure' – ie, average length of time before going phut – is 10,000 hours. Or about five years at six hours a night.
And some machines last a good bit longer than that. A lifetime of 15,000 hours is not unheard of.
And if it ever does need a fix, both ResMed UK and PR UK have a repair department. (When my PR 551 machine conked out after five years, I sent it to them. All it needed was a new blower. And I now have it as my back-up.)
4. You mention being woken up by an auto machine changing pressure. It doesn't work like that. Based on a moving four-minute window, and on what the pressure circuitry detects, the machine goes up in 1.5 cm 'steps'. It doesn't change pressure up suddenly or wildly.
If anything, the ability to respond is – by design – so gradual, that many people find leaving the machine at the ex-factory minimum is not altogether helpful.
So 'best practice', many people find, is to (1) find out your average pressure (while on 'wide-open' auto) over several nights by using the SleepyHead shareware, and then (2) set your minimum to 1 or 2 cm below that average.
But these are just details. Once you have an auto machine, you can come back here for advice on fine tuning pressures.
5. Masks are the most 'varies by the person' bit. Ideally, you would get to try on a range of masks – with the machine running. And that's one of the things you do get as an NHS patient. (At least you do in my area.)
As a private patient and in the UK, the best general advice I can give you is: buy your masks direct from ResMed.
You can also join their user's club, and get along to one of their users' clinics. That way, you can try on a range of masks. Admittedly, they'll all be ResMed masks. But you will get to try them.
Right now, ResMed UK are pushing the Airfit F20 mask. But they still sell the classic Mirage Quattro. Which is widely used - and it may be it's widely-used for a reason. Ie, it's very 'forgiving'. Meaning 'forgiving of facial variation'.
Also, being in their users' club means you get a discount.
6. I honestly think the offer from Jas_williams is a very fair offer. It's a good auto machine, with several useful features. It has only 900 hours on it (remember, average lifetime is more than ten times that).
And it's a very fair price – a brand-new machine from ResMed will cost you £680.
And he lives in the UK. So there won't be any customs duty or palaver.
And I shouldn't really draw your attention to the fact, but you don't need a scrip to buy from him.

So: I can't say for definite if what you have is UARS. And no one can say in advance that APAP will be of help in your case. But it is, I think, the best 'next thing to try'. And a second-user machine at a reasonable cost is a good way to find out.
RB.