The Allies didn't have the capability to do that until the advent of the atomic bomb. I don't think even Churchill would have fought on after losing half the population of London in one night. Mustard gas was the most potent WMD in the Allied arsenal at the time and is relatively easy to protect against, and did not have nearly the same lethal potential as nerve gas which was virtually impossible to defend against (still is for a civilian population).
During the battle of Britain the Luftwaffe dropped 9,000 tons of bombs on London in one month. It would only take 7,000 tons of nerve gas bombs to kill the occupants of 30 cities the size of London.