Nigel Farage, leader of the U.K. Independence Party, a populist movement whose chief aim is to have the U.K. withdraw from the European Union, puts everyone he meets through what he calls “the Farage test.” Most politicians fail it. “They’re all so bloody earnest,” he says. “God help us.”

What does the test entail? “Number one: Would I want to have a drink with them? And number two: Would I employ them?” he explains. “You’ve already been through it,” he tells me with a chuckle. “Provided you get one out of two, you’re OK...