LONDON—On Thursday, Boris Johnson delivered the most chilling warning from a British prime minister since Winston Churchill prepared the country for potential destruction during World War Two. “I must level with you,” Johnson said, looking like a man who had just emerged from a doctor’s office after receiving a terrible diagnosis. “Many more families are going to lose loved ones before their time.” His message was stark, and somehow seemed to hit even harder coming from a man who has built his political career on flippant clownery. The novel coronavirus can no longer be contained in the United Kingdom, he...