I'm just thinking about ways that PR could be passed. If Labour get a massive majority, and the party (ie. MPs) want PR but the government does not, would having a ton of MPs beyond a majority make it easier to pass as a private member's bill in defiance of the government, as even a substantial amount of Labour MPs sticking with the govt would not bring aye votes below the 50% mark? (Plus if the newly strengthened Lib Dems voted in favour)
I mean the big problem is how the labour party will whip.
PR is a significant issue, that changes how much power a party will have in the future. If PR goes through, it's quite likely that no party will ever have enough MPs to rule without support of another party. It's also likely to lead to the larger parties splitting into different factions. Because of this, the labour leadership are going to have strong opinions about it, and if they don't support it, they will probably force their MPs to oppose it by using the party whips.
There's one thing it will do, and that's decrease legitimacy of the current system. If Labour get a supermajority and end up with 70% of the seats on 45% of the vote, it makes it very obvious that first past the post isn't working. With that and what's likely to be every other party calling for voting reform, it does make a cross party consensus on voting reform more likely.