LONDON - Britain ruled out a mission by special forces to rescue a Briton whose life was threatened in the latest execution video released by Islamic State militants, saying on Monday it did not know where he and other hostages were being held.In the video released on Saturday, IS militants threatened to kill taxi driver Alan Henning if British Prime Minister David Cameron continued to support the fight against them.The group has already killed another British hostage and two Americans in barbaric beheadings which have provoked revulsion around the world, using the tactic to put public pressure on Western governments after US air strikes helped halt the IS advance after it swept through northern Iraq.
