It's not dying that scares me

It's the staying dead.

A question about British politics

Now that the Alternative Vote referendum has been held (and failed to reform voting), why are the Liberal Democrats still in coalition with the Conservatives? Couldn't they force a confidence vote, which would fail, and dictate terms to the Labour Party for a new coalition? The Libs can't possibly relish being part of the government which destroys the UK for a generation, can they?

(I assume that enough of the small parties would join Labour and the Lib Dems to form a majority--but maybe I'm completely misreading things and the Conservatives would have an easier time scraping together 20 votes from the small parties than the center-left would have scraping together 11.)