> OK, so the problem is that I find it basically impossible to understand how a well-informed and rational person could come to the conclusions you've come to.
That is indeed a problem, but it is your problem. If 48% of people support anything, there must be some reasons for it.
You're right about the consensus among economists. At the same time, 10 years ago there was a consensus about the virtues of free trade. Then Autor et al. wrote "The China Shock" and other similar papers, and, well, now there's not a consensus any more. Also, economics isn't immune to bias. Are papers showing a link from immigration to low wages, or crime, or reduced trust, likely to get a fair hearing?
Is desire for ethnic homogeneity close to racism? It seems to be very widely shared - including among impeccable liberals. See the evidence on white flight in Kaufmann's Whiteshift. Homophily is close to a human universal, so it's not obvious that it is wrong, or stupid, to prefer others like ourselves.
If we trade we have to accept rules. But now we get to choose which, right? As you say, if we don't like US rules on chicken, we can trade that off against the costs. The value of tying ourselves to the EU depends on how you view its future. There's a fair case that it is an aging, sclerotic continent that can only make money by imposing fines on US tech companies. I don't say that's the only perspective that you can make.
Lastly, the UK is a democracy. The EU, hardly (see Perry Anderson's recent essays in the LRB for a fairly comprehensive account). Democracy sure has its flaws, but doesn't the threat of the boot encourage politicians to get things done? In this context, the contrast between the UK's vaccination program and the EU's is telling.