There were plenty of German air raids on Britain after September 1940. The Blitz, the night bombing of British cities, started during the tail end of the BoB, but continued through May 1941 when Barbarossa forced the relocation of a good chunk of Germany's air assets to the East. Nighttime raids continued on a low-level in 1941. The following year brought about daylight Jabo-raids by single-engined fighter-bombers against targets of opportunity. These pinprick raids were quite small, but they demanded a much larger Allied response to counter raids by ca. 30 FW-190s. Such raids paled before the Allied bombing raids over Germany, and the destruction wrought by these raids prompted Hitler to order the Luftwaffe to begin again a nighttime bombing campaign against the British Isles. Operation Steinbock, sometimes referred to as the "Baby Blitz," lasted from January to May 1944 and targeted British cities. Steinbock failed for a number of reasons, not the least of which was that the Luftwaffe bomber arm had atrophied considerably since the heady days of 1940 and British night defenses were reaching the peak of their effectiveness.

While it made little difference in the course of the war, the post-Blitz attacks on the UK did exact a price in blood and treasure. But aside from the Jabo attacks, it was the Germans who often came out for the worse in these exchanges. British air defenses had made massed conventional daylight attacks too costly, and by 1944, night attacks we now quite costly to undertake.

In regards to second part of the question, yes, reconnaissance flights did get through. It was for this reason that the deception aspect of the Overlord operation was so important. If you have ever seen footage or images of the deception plan, you have probably seen the inflatable trucks, tanks and jeeps that the Allies created to fool the Germans. These were intended to be photographed from the air and were placed in specific locations to make the Germans think that the Allied invasion would occur at Calais rather then Normandy.

The Allies were actually quite effective at preventing German reconnaissance flights, intercepting them before they could complete their mission but they did occasionally allow some flights to slip through with the intention of allowing the Germans to photograph the buildup in areas that the Allies wanted them to see.