Labour have offered PR? Since when? They'd stand the most to lose from it.
1987 General Election
Conservative - FPTP 376 seats; PR 274 seats (-102)
Labour - FPTP 229 seats; PR 200 seats (-29)
Liberal Alliance - FPTP 22 seats; PR 147 seats (+125)
Result - FPTP Conservative majority of 51; PR Hung parliament
1992 General Election
Conservative - FPTP 336 seats; PR 273 seats (-63)
Labour - FPTP 271 seats; PR 224 seats (-47)
Lib Dem - FPTP 20 seats; PR 116 seats (+96)
Result - FPTP Conservative majority of 10; PR Hung parliament
1997 General Election
Labour - FPTP 418 seats; PR 284 seats (-134)
Conservative - FPTP 165 seats; PR 202 seats (+37)
Lib Dem - FPTP 46 seats; PR 110 seats (+64)
Result - FPTP Labour majority of 88; PR Hung parliament
2001 General Election
Labour - FPTP 413 seats; PR 268 seats (-145)
Conservative - FPTP 166 seats; PR 209 seats (+43)
Lib Dem - FPTP 52 seats; PR 121 seats (+69)
Result - FPTP Labour majority of 83; PR Hung parliament
2005 General Election
Labour - FPTP 356 seats; PR 228 seats (-128)
Conservative - FPTP 198 seats; PR 209 seats (+11)
Lib Dem - FPTP 62 seats; PR 143 seats (+81)
Result - FPTP Labour majority of 33; PR Hung parliament
2010 General Election
Conservative - FPTP 305 seats; PR 235 seats (-70)
Labour - FPTP 258 seats; PR 188 seats (-70)
Lib Dem - FPTP 57 seats; PR 150 seats (+93)
Result - FPTP Hung parliament; PR Hung parliament
Generally speaking, whomever is in power has most to lose from PR, but Labour would have had a reduced share under PR in every election since 1959 compared to FPTP whether in power or not. Even the absolute landslide in 1997 - in fact even when Labour nearly managed a 50% vote share in 1945 they'd have lost 76 seats under PR...
I can't see the current Labour party ever approving any attempts at PR - it suits them even less when they're in opposition than in power. To be entirely fair, it's not exactly the Tories' cup of tea either, but they get more seats through PR when they lose which would mean they'd slightly more powerful in opposition at the expense of no-one ever actually being in power.
That aside, I can't see for the life of me how PR can ever actually work grafted onto the current setup. The BNP would, as an example, get 11 MPs returned. Given that no constituency returned them, how will their constituencies be chosen? Which 11 members of the BNP would be chosen as the MPs and by whom? At the less extreme end, where would the 93 extra Lib Dem MPs come from?
I think we need to get away from the entire concept of constituencies and local MPs representing our interests nationally. After all, our local interests are best served... well... locally. And we have local councillors for that - and this system confused many people. The sheer number of people who didn't get why they were voting for two different people (local elections and parliamentary elections) in some areas on Thursday is staggering and I can't say I blame them.
Perhaps then, once we've got local issues addressed locally, by local councils and councillors, PR can select MPs to deal with national issues nationally - and we'd need fewer of them (our lower house has 650 members; the American lower house has 453 members - our upper house has 733 unelected members; the American upper house has 100 elected members; why do we need so many more people in charge nationally for a considerably smaller and less populated country?).