Open this photo in gallery: Labour Party Leader Keir Starmer meets staff at a supermarket while on the general election campaign trail, in Wiltshire, U.K., on June 19.Stefan Rousseau/The Associated Press Britain’s July 4 election is a contest between two politically unexciting and not very eloquent men, both of whom have spent the past few years struggling to reunite once-mighty political parties that had torn themselves apart. Surprisingly, the one on the path to a generational majority shift is not Rishi Sunak’s Conservatives but Keir Starmer’s Labour Party. To understand how, you need to gaze deep into the radioactive cores...