I think mumsnet has been a coordination point, for sure. And while anonymity has downsides in the kinds of beahviour it seems to create at times, on this issue it allowed people to talk much more openly than they felt they could in places where they were named. At one time, really contentious social issues were talked about more openly in certain real, public places. Even when I was in high school I remember really serious discussions on abortion, that were much more sophisticated than what I see in the press today. And also in universities. And the same for other issues - universities have stopped being places where people can really dig their teeth into such questions, work out the problems. Which happens here, within the discussion where there is disagreement difficult problems get raked over, worked out, elucidated. But I think only because we can't really threaten to call people out with all the modern consequences.
That being said, I do think there are other issues. The fact that in the US political partisanship is so reflexive is a problem. Here in Canada it's also that many of us reflexively define ourselves against Americans. We are also ill served in terms of our press, mainly because of our small numbers - there just isn't the multiplicity of editorial views.
And then I would also say, there is a lot that I suspect comes down to the different political traditions. The US, Canada, places like Australia - our politics was born in the Enlightenment, at a time when people began to believe it was possible to construct a whole new political order. Liberalism was born there. While the UK has a liberal tradition it was built on top of a well established conservative tradition that has a kind of continuity or development that goes back 1000 years. Even the left in the UK is rooted in that tradition which I think has very different foundations than the politics of the Enlightenment. It's based on a much more organic sense of the person and family and maybe place - less abstract.
The British system seems more inclined to take a Chesterton't fence approach. And also it may be significant that in places like Canada and the US, documents like the Constitution can over-ride the will of the House, as can the courts, in a way that is not the case in the UK.
I also think colonialism plays a role in places like Canada but I am not really sure what the link is.