The 2011 referendum did not offer the choice of PR - only AV which is mot a proportional system. Why should a small minority of voters in marginal seats be able to inflict a dreadful government on the rest of us?
There is an official E-petition asking for consideration of PR. petition.parliament.uk/petitions/168657
Please sign the petition and spread the word if you believe PR would be better than FPTP. It probably won't get anywhere but Labour and the minority parties support the idea of PR and even some Tories are coming round the idea of fairness.