A New York-born playwright is at the center of a British political scandal that could decide the country’s next prime minister. Eve Leigh — the lefty 34-year-old daughter of famed Broadway composer Mitch Leigh — and her husband admitted Saturday that they called the cops Friday about the plate-smashing row between frontrunner Boris Johnson and his live-in gal pal, Carrie Symonds, and then tipped off the press. News of the spat — during which Symonds, 31, allegedly asked Johnson to “get off me” and “get out of my flat” — has dominated UK politics in the days since, with Johnson,...