By Elizabeth Howcroft LONDON (Reuters) – The British pound fell on Monday to its lowest in three weeks versus the U.S. dollar, as investors sold riskier assets amid expectations of Fed tightening, mounting tensions with Russia and falling stocks. The market is bracing for a U.S. Federal Reserve meeting this week that could signal the removal of its vast stimulus programme. The United States also said on Sunday it was ordering diplomats’ family members to leave Ukraine, in one of the clearest signs yet that Washington is bracing for an aggressive Russian move in the region. The dollar rose and …