Who broke Britain? Someone—or something—must have. The past 18 years, enough time for a whole lost generation to be born and brought up, have yielded nothing but stagnation and mass disillusionment. In 2007, before the global financial crisis, Britain was at its postimperial zenith. Median household income had just surpassed that of Germany. A pound was worth more than $2, and London was arguably displacing New York as the center of international banking. But since then, Britain has been left behind. The country’s output per person is now only just above that of Mississippi, America’s poorest state—and that slight lead...