Forget the American electorate. Donald Trump’s biggest fan may be a British businessman who likes to play the odds. John Mappin, a media investor who owns Camelot Castle in Cornwall, has placed more than 15 wagers, totaling more than 2,000 pounds ($3,100), that Trump will secure the Republican presidential nomination, if not win the presidential election outright. Depending on the type of bet (nomination or presidential victory) — and the odds at the time it was placed — the payoffs range from 7-to-1 to more than 30-to-1. If Trump indeed becomes president and Mappin wins every bet he’s made, he...