First past the post is a one-party system

If to be politically left-wing is the exact opposite of being right-wing then first past the post would pose no problem because each side would have a party to vote for. But the right is intrinsically less collectivist meaning that for it to vote for a single entity is more problematic and less likely to happen. So then the right loses votes because of a system which rewards only the winning candidate. To use first past the post is not offensive to the left in the same way that it is offensive to the right and so then it is a kind of subsidy to the left. Fptp is helpful to the left because left-wing voters do not mind voting for Labour in the same way that right-wing voters might object to voting for the Conservatives. First past the post gives the right only one (realistic) choice with which to oppose Labour which is a problem. The left don't mind so much being forced to vote for the Labour party to stop the right. The fptp system splits the right-wing vote more than it does for the left which means it is helpful to the left. To give the liberal right a better chance requires that a proportional system is used. Without pr the left will have an inherent advantage because the right are more offended by voting for a single party than the left are.