The Battle of Britain, from July -September 1940, was the turning point of the opening of World War II. If the United Kingdom had fallen, the Germans would have been able to throw all their weight into the invasion of the Soviet Union, and the Normandy invasion would have to have been staged from the United States or North Africa. Britain standing alone inspired the American public, and ensured the defeat of Germany, as Hitler's war machine could not stand up to a two-front war.
IWM Duxford has an entire hangar dedicated to the Battle of Britain, with much equipment and vehicles that made her survival possible. There are rare examples of the equipment created to defend herself when Britain expected an invasion that summer.