LONDON—It’s been a long, hot summer for Nigel Farage. Britain’s leading populist has been battered by a string of scandals over political donations. His Reform UK party lost its lead in the polls. And this Thursday he faces an election for his own parliamentary seat against a man who goes by Count Binface and wears a trash can on his head. Farage was widely expected to win his Clacton district in eastern England and return to Parliament. But his political woes are unlikely to end there. Investigations by a parliamentary watchdog and the police over gifts and millions of dollars…