LONDON (AP) — The British government faces weeks of uncertainty as embattled Prime Minister Keir Starmer prepares for a leadership challenge from the popular mayor of Greater Manchester, Andy Burnham, who can’t formally launch his bid until he finds a way back into Parliament. Burnham’s path to Westminster is far from certain. He will first have to overcome a strong challenge from the anti-immigrant Reform UK party in a special election for the parliamentary seat that was vacated to make way for him. British government borrowing costs rose Friday and the pound weakened on investor concern about continued disarray at the heart of government. The pound has dropped 1.4% against the U.S dollar this week. Weeks of speculation about Starmer ’s future broke into open rebellion within the governing Labour Party on Thursday as Burnham declared his intention to seek the top job and two other senior members positioned themselves for their own bids.…