The first past the post system is stacked against the opposition. Also it is not representative, didn't UKIP get 1 million votes but zero seats quite recently? As much as I disagree write their party, those voters not being represented what causes disenfranchised and angry population. This is partly why the Brexit referendum, for example, was such a protest vote.
@sun_tzu
On another note, every non Tory party and the population as a whole would benefit from PR. It is more representative.
All parties should band together, so in each constituency, have only one non Tory MP, standing against the a Tories on the sole platform of voting in PR then calling another election.