Morlock, in the comments here, made me think: if PR makes it more difficult for parties to lurch to the left or the right, then maybe Cameron could actually achieve something useful, if only out of necessity.
The thought (such as it is!) goes like this:
Britain is in a financial crisis. The state is pissing away money faster than it can print it. Cameron has little option but to cut up the credit card and get the "swingeing cuts" axe out.
Blood starts pouring, the state gets 25% smaller (not enough, but hey ho!), people start getting angry, more cuts are needed, Cameron has the taste for power and so offers a PR referendum to the Limp Dumbs in exchange for propping him up for another couple of years.
The Limp Dumbs agree, Cameron cuts the state even more and in a rage against the Tories, the public votes overwhelming for PR.
The state is now half the size it was before, very manageable and the ungrateful public now lock the political model in at that level.
And then I woke up and my coffee was cold.