The last time there was a UK Prime Minister who wasn’t Labour or Conservative was David Lloyd George in 1922, when the Labour party barely existed, then Labour overtook the Liberals and has never relinquished its position as one of the two major parties. Before that the two parties that swapped back and forth were the Conservatives and the Liberals and before that the Tories and the Whigs (really, the same parties with different names).
Yes, there are lots of regional parties that do well (esp. the SNP in Scotland right now), but there’s only three national parties of any significance, and one of those is all-but dead right now (again). (I’m ignoring UKIP, because fuck 'em, and they can’t win shit).
This doesn’t represent the diverse views in the UK. FPTP skews the results horribly. Proportional representation would provide a much broader sweep of parties with seats in Westminster, and we’d have coalition government all the time.
The UK is better than the US in terms of third parties getting representation, but not by nearly enough.
