You know we have a coalition government with a third party at the moment, right?
I voted against PR in the referendum - don't wanna get too off topic by going in to it. And yeah, there is a common opinion that the two major parties are similar, not sure how true it is but no real interest in holding that debate. But the very fact that we have three main political parties makes it much more open than I get the sense the US is - just my impression, Americans, no offense if I'm wrong. It feels like a third party candidate is almost a joke candidate at the moment while we have three major parties and a host of other minor but influential ones - the Greens have an MP, UKIP are important on a European stage, the BNP do well in some local elections. Yeah, none of those I'd want in power but as I say, my impression is that we're way more diverse politcal party wise than the US
EDIT: Just thought - thats not even getting into the horrific can o' worms that is Northern Irish politics and is also ignoring the strength of the SNP and Plaid Cymru.