Fantastic question. They lost the battle of Britain for the most part because they underestimated the British's sophisticated air control radars and the RAF itself. The spitfire and Hurricane (Even though it was starting to show its age) were fantastic aircraft, and combined with British air controllers guided by radar it had an enormous impact on German air raids, it was not possible for the Germans to surprise the British, instead every single raid was tracked and fighters dispatched with exceptional accuracy and intelligence on every raid. The Germans also changed targets often and once they decided to raid British cities instead of military targets, that was the end of it for the most part. Eagle day (the last massive German air offensive) saw some serious German air crew and aircraft casualties, casualties they could not afford as they built up for Barbarossa.