LONDON — He’s branded one U.S. ally's government “fascists” and another leader a “fool.” Now, Elon Musk has labeled Britain a “tyrannical police state” while endorsing calls for a new election and boosting a video from a jailed far-right activist. Musk, not just the world’s richest man but a key adviser to President-elect Donald Trump, has clashed for months with Prime Minister Keir Starmer at a time when the U.K. is concerned about its standing with the incoming U.S. administration. Over the weekend the tech billionaire took his feud to a new level, taunting the U.K.'s new center-left leader over...