The reason the parties are divided on this (main stream ones) are because of what they will/wont gain from it.
Lib Dems will win more seats in theory from it
Labour want PR in parliament and see this as a side distraction (fair point but pr is a ball ache anyway)
Conservatives will loose several safe seats, St Albans being a primary example (where I am now) as the local Tory seat is able to hold out against a divided Labour Lib Dem vote.
The fringe parties either want to keep what we have now so come their Reich they can abuse it or want PR to get silly top up seats or something.
Labour absolutely do not want PR, they are the ones who benefit most from the current system, remember that Labour got a lower percentage of the vote in 2005 than Cameron did last year, and they also got a lower percentage last year than Major did in 1997.
Why do you think Labour are so against fixing the constituencies to eliminate the current significant labour bias?