The problem is that it always makes the people who are most affected by it (and thus least powerful) in favour of PR, and the people who take advantage of it are against it.

So you need to end up with a coalition where one of the members is affected by it. And then to actually either get the law passed or to win a referendum.

All of which is possible. But I don't think Labour are going to voluntarily give up their power unless forced into it in some way.

(I'd love to be proven wrong!)