Proportional rep. would make it easier for the right

First past the post is good news for Labour (the generic party of the centre-left) because it means they must only defeat the Tories. Pr would mean Labour would need to defeat not just the Tories but all other parties of the right... which would be much more difficult. Beating the Conservative party is relatively easy for Labour to achieve under first past the post. Pr would make it more difficult for Labour and easier for the right.