UK is not a good counterexample here. It also uses the first-past the post voting system, which trends towards two parties since any third party becomes a spoil

UK is not a good counterexample here. It also uses the first-past the post voting system, which trends towards two parties since any third party becomes a spoiler. With a different voting system there would be a wider range of smaller parties, so you don't need to wrest control of a major party to be represented.