The answer to this question can only be a hypothesis as we will never know the true answer as we are looking in heinsight at what could have happened if Britain did not have radar.
Though the Battle of Britain did not occur until the summer of 1940, in many ways Britain's preparation for the battle began in the early 1920s when the threat of invasion first raised its head - not from Germany, but from Britain's traditional enemy, France. In 1922, the French Armée de l'Air had a striking force of 300 bombers and 300 fighters poised across the Channel.
A committee created by Air Commodore Steel and Colonel Bartholomew in 1923 produced a plan of defense based on the assumption that any attack on Britain would come from France and concentrate on the South coast. The plan consisted of a defensive belt 15 miles thick between Duxford and Devizes, called the Air Fighting Zone, with anti-aircraft artillery and searchlights arranged along the belt. Sound Locators and Observer Posts were constructed on the fringes of the belt, allowing the RAF Advanced warning of an attack. Although by 1940 the increased speed of fighter aircraft meant that the Sound Detectors would only give a minute's warning before an attack, it paved the way for a more effective early warning system: radar. The plan also correctly predicted where the enemy attack would come from, and what its main targets would be; the only difference was that the airmen in the aircraft coming from France were German, not French.
