By Saikat Chatterjee and Joice Alves LONDON (Reuters) -Sterling was volatile on Tuesday after falling to a near three-week low against the U.S. dollar on Tuesday, with investors on edge a day after British Prime Minister Boris Johnson survived a confidence vote that left him politically weakened. Johnson won the vote by 211 votes to 148, seeing off a challenge to his leadership brought by lawmakers from his Conservative Party. But the larger than anticipated rebellion leaves him battling to win back the confidence of his colleagues and the public. Heightened political uncertainty dealt sterlin…