From Wikipedia
15 August was “The Greatest Day” when the Luftwaffe mounted the largest number of sorties of the campaign. Luftflotte 5 attacked the north of England. Believing Fighter Command strength to be concentrated in the south, raiding forces from Denmark and Norway ran into unexpectedly strong resistance. Air Marshal Downding put up nine fighter squadrons in Norhern England and Scotland for reserve and rest pilots and German bomber formations expected from north due to Ultra intelligence, spotted and trackerd by radars the moment the were in range and eight fighter squadrons scrambled. Inadequately escorted by Bf 110s, bombers were shot down in large numbers. North East England was attacked by 65 Heinkel 111s escorted by 34 Messerschmitt 110s, and RAF Great Driffield was attacked by 50 unescorted Junkers 88s. Out of 115 German bombers and 35 fighters sent from Luftflotte 5, 24 bombers and seven fighters were shot down (about %20 percent casaulty rate)
In further south Luftflotte 2 and 3 were more sucessful , overwhelming RAF air defences and hitting three airfields belonging Group 11 and a port in Southeast England but attackers casaulties were also heavy due to constant RAF fighter interception of both incoming bombing run and return flights of Luftwaffe groups despite German fighter escorts presence. In total during Greatest Day in total 75 German planes were destroyed and many others damaged beyond repair. Furthermore, due to early engagement by RAF fighters many of the bombers dropped their payloads ineffectively early. As a result of these casualties, Luftflotte 5 did not appear in strength again in the campaign. Only 27 RAF fighters shot down that day and of these 17 of RAF pilots parachuted safely to return back to action
Following day , Winston Churchill visited RAF Fighter Group 11 Headquarters at Uxbridge and followed engagements and quoted "Never was so much owed by so many to so few*