actually there was another defensive move used by Bomber command that did involve speed. If an attacking fighter was coming in for a pass the tial gunner could call "drop" if he did the flight engineer would drop the throttles (the pilot didn't controll the throttles) and the bomber, ,let's assume it's a lanc, would rapidly loose speed, this caused the gap between fighter and bomber to drop allot quicker than the fighter pilot would have planned, as such he wouldn't be able to get any shots in and the tail gunner would have a few shots, upon hearing the command "easy" from the tail gunner, the flight engineer would increse the throttles, this whole process would be over in a matter of seconds..............