LONDON — After a whirlwind two months packed with drama and crisis, Britain finds itself right back where it was before with some of the same faces competing to become the third prime minister in just eight weeks. Supporters for the three presumed front-runners — Rishi Sunak, Penny Mordaunt and, yes, Boris Johnson — were out of the blocks early on Friday, setting out their pitches for why their person should get the keys to 10 Downing Street, the prime ministerial residence. Could Boris Johnson stage an extraordinary political comeback? What about Rishi Sunak, the bookies’ favorite who fell to...