Yes, I did buy it from eBay. The seller was from the USA, and did have a disclaimer saying 'These may or may not work internationally, use at your own risk', but then the auction was advertised worldwide (otherwise I wouldn't have found it). I checked their feedback first and found people from all over the world, including the UK, have bought them from this seller and used them with success.
Also I would think that nationality shouldn't be a problem. Surely you could create an account and pay for a Gold subscription in the USA, then move or go on holiday to the UK and use your account there? The internet is global, after all. The only thing is, because 12 months of Live is $50 (£25) in the USA and £40(!) in the UK, Microsoft may be funny about paying for a US account in the UK. But apart from that I don't see why it should matter.
And, besides, I've checked the official support forum and other people are having the same problem regardless of payment method and locality. A Microsoft representative told me that the problem is not localised and that it's most likely a corrupt profile on their servers.
What I'm going to do is hold out until Monday, when they're performing some maintenance on Live, and see if that fixes it. If not, I'll phone them and complain some more!