Prime Minister Sir Keir Starmer giving a speech at the Coin Street Neighbourhood Centre in Waterloo, London. James Manning - Pa Images | Pa Images | Getty Images Bond traders are bracing for more U.K. instability as Prime Minister Keir Starmer could be formally challenged by rivals on Thursday, as he clings to power. Health Secretary Wes Streeting is expected to resign to launch a leadership bid, while Starmer's former deputy Angela Rayner has reportedly been cleared of deliberate wrongdoing over her tax affairs, boosting her prospects for another potential leadership bid. Supporters of a popular third contender, Greater Manchester Mayor Andy Burnham, are said to pressing the Labour Party's governing body to extend the timeline for any forthcoming leadership election so that the he can seek the seat in parliament he would need to run for the leadership.…