(Reuters) - British shares slipped on Wednesday as tensions in the Middle East threatened to boil over after Iran retaliated by launching missile strikes at U.S. forces in Iraq, driving investors away from risky assets and into safe-haven holdings.The FTSE 100 fell 0.4% and the FTSE 250 shed 0.7% following news that Iran has hit back in response to the killing of a top Iranian commander by the United States last week.Anglo American led losses on the main board, dropping 2% after saying it was..