Complex sentence: The British Electoral System, known as First Past the Post, in which seats are filled on a constituency basis, is unproportional, ie, it does not give parties a parliamentary party in size related to the percentage vote they received.
Split into simpler sentences: The British Electoral System, known as First Past the Post, is non-proportional, and seats are filled on a constituency basis. This lack of proportional representation means it does not give parties a parliamentary party size directly related to the percentage vote they received.