They are already in the mainstream. Social media and brexit has made sure of that. The overton window has already moved in the last 6 years. But the current FPTP system weakens the progressive parties and amplifies the right especially the Tories.
Im sure you know my thoughts on Brexit. But like what I’m saying to Scara about Overton’s theory. The ground swell of support for Brexit didn’t appear accidentally, nor was it spontaneously manifested. A decade or so ago most people had no strong view either way. Giving Farage et al airtime and therefore legitimising their view point amongst other techniques led to that shift if the window. Also wasn’t electing your EuroMP done through proportional representation? If I remember correctly and it was, considering the votes UKIP got and what that led to, it is another reason to keep FBTP.
So Brexit has happened and we have a government that is pandering to the worst parts of our society with thinly veiled nods to racism and distain for the working class, it’s happened already so why not allow PR which sounds like a fairer system… well you are making the assumption that this is as bad as it gets. It is not, It can and will be worse if we are not watchful.