It's odd that these sorts of riots are happening right now in England if the mistrust into the political system isn't so high. I suppose the rioters don't represent a significant voting constituency that would actually affect that sort of thing.
I think that the reason the UK has two major parties is that they're stuck with them due to their electoral system. Sure, a two-party system can create less extremism within governments, but it also creates a stagnation of new ideas and makes people outside of the two parties feel totally unrepresented. In the US, we're suffering from the drawback of the two-party system... we replace a bad government with another bad government because that's our only alternative. I would be willing to go to PR just to allow 3rd party voices in the government (although I'm probably biased, seeing that I frequently vote 3rd party). At least in the UK, they got a coalition, which I think is better than the Conservatives alone. The Lib Dems have helped the direction of the coalition.
If the UK got PR right now, I don't think they'd elect BNP or even UKIP significantly. The Greens are the most likely to grow IMHO (and Labour would probably make the same number of gains they're likely under the current system). The UK would probably get along about how they have been. So I think it's more of an attitude amongst the voters rather than the political system itself.