LONDON — When Boris Johnson won a landslide election victory for his Conservative Party in 2019, he loomed as a colossus over British politics, the man who had redrawn the country’s political map with a vow to “get Brexit done.”

With an 80-seat majority in Parliament, the greatest amassed by a Conservative leader since Margaret Thatcher in 1987, Mr. Johnson seemed assured of five years in power. Some analysts predicted a comfortable decade in 10 Downing Street for Mr. Johnson, the…