LONDON (Reuters) -After protesters hounded Britain’s Labour leader Keir Starmer, Prime Minister Boris Johnson faced pressure on Tuesday to withdraw a claim that the opposition leader had failed to prosecute one of the country’s most notorious child abusers. Johnson, who won a landslide in a 2019 election, is facing the gravest crisis of his premiership after a series of scandals including revelations that he and his staff attended Downing Street parties during COVID lockdowns. As Johnson apologised to parliament for the parties on Jan. 31, he falsely claimed Starmer had failed to prosecute Jim…