political idealism doesn't resonate with most Brits.
Technocratic arguments resonate even less. Partly because post-WMD and both-crash no-one believes a word that government "experts" say. But mostly because it's bollocks: for many Indians, remaining as a British colony would have been economically sensible in the short to medium term. People connect with ideas, and telling them that it doesn't matter if things they don't like happen so long as food is a few percent cheaper doesn't help.
Here's a thought experiment. Suppose I could prove to you that something right thinking people would regard as utterly unacceptable (say, expelling anyone whose grandparents were not British citizens) would make housing affordable, create jobs and improve the NHS. Thought experiment, obviously: it's bollocks, of course. Would that justify voting for it?
All over the US, poor Republican voters vote repeatedly for grim despots who send their children to pointless wars, allow their jobs and futures to stagnate and deny them healthcare. But so long as guns are as legal as they want abortion to be illegal, and them dang godless homosexualists can't marry, it's all good. They are voting against their economic interests (I was going to say "class interests" but I'm not selling Socialist Worker) but for "values" and "beliefs".
We are committing political suicide if we believe that, outside the audience at an Eddie Izzard gig London, there are not a lot of people who would happily see themselves a bit, or quite a lot, poorer so long as they could have back the social cohesion that they believe was present in some imagined white, wife-beating golden age.