Why are British eurosceptics opposed to Proportional Representation?

Why is it that Britain, possibly the most eurosceptic country in the EU, does not have a substantial “Leave the EU” party in the House of Commons? The answer, of course, is that the electoral system does not permit it. But what is truly baffling to this non-Brit is the way that hardline eurosceptics conspire with pro-European Tories to keep first past the post, and then complain when the pro-European Tories get elected and do pro-European things. Why are hardline eurosceptics not advocating changing the voting system to one where their votes actually matter?