Conservative seats won: 306
Labour seats won: 258
Liberal Democrat seats won: 57
Votes cast per Conservative seat: 35,021
Votes cast per Labour seat: 33,338
Votes case per Liberal Democrat seat: 119,397
If the seats had been divided in proportion to votes cast:
Liberal Democrat: 149
UK Independence Party: 20
British National Party: 12
Scottish Nationalist Party: 11
One of the effects of PR would be to give the BNP seats in parliament, which is a slightly unnerving thought. But then, since they represent a proportion of the electorate, where their policies aren't actually illegal, a liberal society has to be prepared to give them a voice.
It is very apparent looking at these figures, how strongly opposed the two largest parties will be to electoral reform, which would completely undermine their ability to dominate the political agenda for generations. But it is also very apparent that the fact that one political party can run a government and control the political agenda for 5 years on the basis of less than a third of the popular vote has nothing to do with democracy. Also, if you don't vote for the MP that gets elected in your constituency, your vote is completely irrelevant in the current system - you might as well not have voted. And in most constituencies, one party has such a dominant position that anybody with a different political opinion is in effect disenfranchised.
Sources: Twitter (Rillaith, GdnPolitics), BBC