LONDON (Reuters) - British police arrested two men as part of an operation into Islamist-related militancy on Friday as lawmakers prepared to approve Prime Minister David Cameron's plan to join U.S.-led air strikes against Islamic State militants in Iraq. Britain last month raised its international threat level to the second-highest classification of "severe", meaning an attack is considered highly likely, and Cameron has said Islamic State militants battling for territory in Syria and Iraq pose a grave security risk. ...