The Labour party and the Tories have shared interests

The nature of the first-past-the-post (fptp) voting system is that (at least initially) it tends to result in a two-party system. But over time we see that it is more favourable to the party of the left because voters on the right are more inclined to reject a monopolistic party (they want more choice). So whilst (in the UK) the Tory party might consider themselves to be beneficiaries of the fptp system in fact their voters lose out. Assuming voters on the right are more interested in policies than parties it is clear that fptp is detrimental to their interests.

First-past-the-post is very helpful to Labour (more so than to the Tories) in which case Tory support for fptp can be interpreted as a subsidy for the Labour party. The Tories could easily damage the Labour party by advocating a form of proportional representation but since they don't we deduce that the Tories support Labour. Antagonistic parties would not support each other in this way and so then it is clear that the Labour party and the Conservative party have shared interests.