Hitler was enraged by the attack on Berlin and because it seemed that the attacks on airfields were not destroying enough RAF fighters, he ordered a change of targets. By attacking cities and industry, the Germans hoped to break British morale and to destroy the factories that built fighter aircraft. They also hoped that RAF fighters would gather in force round the cities to protect them, which would make it easier for the Luftwaffe to shoot them down in the numbers required to establish air superiority.
The change of plan was a mistake for a number of reasons. It gave 11 Group a chance to repair their airfields and radar sites, so the defences became fully operational again. The German Me 109 fighter could only carry enough fuel for 20 minutes flight over Britain, so London was on the edge of its limited range. Finally, the German raids now came within the range of 12 Group, and their large formation tactics known as "Big Wings".
Much has been written about the different tactics employed by Nos 11 and 12 Groups and their commanders, and the supposed disagreements these differences caused. Suffice to say that 11 Group's fast response tactics with whatever was available, meeting the enemy formations as far from their targets as possible, was best suited to their geographical proximity to the German bases. 11 Group Squadrons simply did not have the time to assemble, they had to get airborne and climb to height as quickly as possible or miss intercepting the raid altogether. 12 Group, being further north had somewhat more time for a large formation of fighters to assemble and climb to meet the oncoming attacks, tactics that suited their circumstances. Dowding, as befits a true leader, allowed his Group commanders to run their organisations as they saw fit, the detail work being done a Group level while he dealt with the overall picture. The life of an 11 Group pilot was made more difficult by these operating methods, but Park understood the true situation of his command, and employed his Squadrons with brilliant effectiveness. In the light of the outcome of the Battle, and the fact that for many days he had the fate of a nation resting on his shoulders alone, Park must be considered as the architect of the RAF's victory.
Knowing the target to be London and the industrial centers, the British controllers now had time to assemble a large number of fighters to attack the German formations and break them up before they could bomb. The appearance of large numbers of Hurricanes and Spitfires came as something as a shock to the Luftwaffe pilots, who had been told by their intelligence officers that Fighter Command had practically been wiped out by the earlier raids against the airfields. By changing tactics and targets, the Germans had actually helped Fighter Command to deal with raids.
For the people living in the cities, the Blitz had begun, as night raids followed daytime raids and gave the civilians little rest. Everybody was in the front line, and there was little the RAF could do to stop the night raids. Airborne radar was in its infancy, but there were some successes for the Blenheim, Defiant and early Beaufighter night-fighter Squadrons. Some of the Hurricane and Spitfire day-fighter Squadrons also took part in the night defences, but relied largely on luck to make an interception.