The left are the lesser of two evils in a fptp system

The Labour party and any generic party of the centre-left (such as the Democrats in America) do well under fptp because of tactical voting... the Labour party are less hated than their opponents which means they do better in fptp elections. Because of tactical voting fptp elections will mean that the Labour party generally does better than the Tories. This does not apply however in pr elections where tactical voting is not required as we can generally find a party which reflects our views.