The Tory party is no longer the opposition

Provided the taxes imposed by the (Labour) party are not too high it is consistent to vote Labour to oppress the Tories for their position on fptp. Even though Labour too are opposed to pr (otherwise it would not even be an issue) it is consistent to vote for them because pr would reduce the size of the state. A large and oppressive state is not a contradiction for the 'left' and so we can understand why they are in favour of... or at least tolerate... fptp. By voting Labour we are punishing the Tories for being inconsistent in their support for fptp. If the Tories (still) support fptp then it makes sense to oppress them by voting for Labour. Even if Labour are to the left of the Tories they are not significantly to the left to the extent that we are fearful of the difference between them and the Tories... the Tories are just as bad in terms of taxation. By voting for Labour the Tories will eventually realise that fptp isn't working for them.