> The UK is Facebook mad, of course, but doesn't really have a locally popular equivalent of Reddit, Hacker News, Digg, MetaFilter, etc.

There are plenty of UK people who read these. I expect the reason there aren't any popular UK-specific equivalents is simply that most English-speaking people on the net aren't from the UK, so any English-language site is likely to be dominated by non-UK people (unless its content is UK-specific).