Election night in Britain used to be the Dimbleby family business. Former war correspondent Richard Dimbleby hosted the BBC’s first all-night results coverage on television in 1955. His eldest son David Dimbleby hosted 10 in a row from 1979 to 2017. For three of those – Tony Blair’s hat-trick of New Labour victories from 1997 – David’s rival for ratings was his younger brother, Jonathan Dimbleby, who anchored ITV’s coverage throughout the night. When the UK goes to the polls next month there will be a Dimbleby-shaped hole in Britons’ election night experience for only the second time in 45...