British Prime Minister Sir Keir Starmer is struggling to hold on to power after the resignation of  his main rival in the government, and others position themselves for potential challenges.  Disastrous results for the governing Labour Party in local elections last week have plunged Britain into a new crisis, just under two years after Starmer won a large majority on a vow to bring stability and end a decade of political chaos. After days of calls by Labour MPs for Starmer, 61, to quit or set out a timetable for his departure, Wes Streeting resigned as health minister, on Thursday, the first senior minister to break cover. The 43-year-old said he was standing down because "it is now clear you will not lead the Labour Party into the next general election". While Streeting did not trigger a formal contest, potential rivals to the Prime Minster, such as Greater Manchester mayor Andy Burnham, 56, started positioning themselves for a possible leadership challenge that Starmer has said he will fight.…