But in the UK do we have to get a Sky HDTV thing to actually watch HDTV?
No, you don't *have* to, but they're bigger than everyone else by a country mile, so it might be the best plan if you're going to get HDTV at all. There was a time when NTL looked like they might be able to take Sky on, but after all their financial troubles, that doesn't seem likely any more.