House of Lords, mainly. The upper house is much less partisan than the Commons and despite its unelected nature (which I, of course, despise) actually does a decent job of reining in the excesses of whatever party sits to the Speaker's right at the time.
There is separation between legislative and executive, as there is in the USA. Laws passed in Parliament are subject to judicial review to make sure they're actually legal.
Oh, and the most vicious press in the world :-)