I made a thread about that, you know

Yeah, it's incredibly frustrating, but those questions asked by me, Xavier (well not really) I can answer:
1. As far as I know: you can buy DLC worldwide, provided you have a credit card working for that region. If you can't get your credit card accepted to pay at the UK store, you won't be able to buy the DLC. But if you buy it, it will work.
2. Yes, you can buy DLC content without having the game it's made for. Don't know why though, you will be unable to play it, as it requires the full game.