LONDON — Thirty-five police cadets stood at attention, a perfect made-for-TV backdrop, as Prime Minister Boris Johnson strayed from a police recruitment announcement to trash his political opponents for opposing his Brexit plans and for plotting to keep Britain “incarcerated” in the European Union. American politicians have long used soldiers and police as props during political speeches. But in Britain, politicians have avoided such staging as inappropriate and tacky — until the arrival of Johnson, the country’s convention-buster in chief. His tactics have repeatedly defied tradition and precedent in a society that treasures yesterday as a guide for tomorrow. Britain famously has no written constitution. Its fundamental guide is not a document but an ethos — a centuries-long history of laws, legal rulings and traditions combined with a collective national assumption about the way things should be done. In his six weeks in office, Johnson has detonated many of those assumptions in his effort to keep his promise to lead Britain out of the European Union by Oct. 31. He ordered Parliament closed for five weeks. Suspensions themselves are not unusual but typically are not for that l...