I'll assume you mean during World War II. Britain had a number of weapons that it used during that war, including the Blenheim Bomber, the Hurricane, the Spitfire (heroes of the Battle of Britain), the Sten submachine gun, and he Churchill and Challenger tanks.
Hard to argue which was the most effective, as they were used in different ways at different points of the war. I guess I would have to give the title to the Spitfire, which came into production just in time to hold off the German air force during the Battle of Britain. Almost anyone who's ever flown one has written or spoke of how she handled like a "kite".