There is
Labourhome (see
http://en.wikipedia.org/wiki/Labourhome for an 'independent' description of it), but that's aimed more at people active in the Labour party (though it's not owned by the party itself). Its format is more like DailyKos than DU, too.
The Lib Dem equivalent of that is probably
Liberal Democrat Voice, but the forum there is for party members only.
If you're interested in reading a fairly wide-ranging left-of-centre site on UK politics (including international politics from a UK point of view), I'd recommend the group blog
Liberal Conspiracy. It pulls together several UK leftish political blogs, some Labour ('old', rather than 'New Labour' Blair types), some Lib Dem, some unaffiliated. You can't start your own stuff on that, of course, just comment in the blog threads they start.
And there's the Guardian Comment Is Free pages too, which are more or less a group blog (all the opinion pieces in the paper get posted there, and some online-only articles too); but you have to fight with RW trolls from all over the world there.