Intelligence gathering. Once the war came into full swing the intel branches of the armed forces taking place went into full swing, whatever that entailed. The Oslo Report and such intel coups like it revealed enormous amounts of information because, although some of what information the report offered was a red herring, it led intel experts to look for more in specific places. British intelligence became aware of German jet fighter projects as early as mid 1940 when the designation He 280 became known through various circles. POWs also contributed, through direct interrogation and through passively listening in to conversations held between captured aircrews. Photo recon grew in importance of course and before the war, the Luftwaffe had the biggest and best photo recon fleet and resource of any air force in the world and, with the strategic reconnaissance unit governed by the Intelligence branch under Wilhelm Canaris in the Bendler Block and not under the auspices of Goering in Wilhelmstrasse, was not subject to the vagaries of the internal politics of the RLM.
Prior to WW2 Britain had to rely surreptitiously on the wily Australian Sydney Cotton for strategic recon, whereas the Germans had an independent specific unit and command structure at its disposal, so was able to photograph British, French and other European airfields at will and unnoticed prior to the war. Once the fighting started, British airspace became effectively shut down to solo recon machines and less information came out, and this is where Britain took the lead in intel. The RAF PRUs revealed the Me 163 in 1942, the Me 262 in 1943 and the V2 rocket the same year. Hard intel on German jet projects was only a matter of time.
Intelligence is, of course only as effective as how it is acted on and it's interesting looking at how the Bf 109F was regarded in Britain as to seeing this at work. The British believed the Spitfire V was its superior, even though it wasn't outright - the Spitfire could of course out turn the F, but that was it - the Messerschmitt was largely superior in performance, but the British didn't really regard its debut with much fan fare when it was encountered in combat, unlike the Fw 190 and it was that aircraft that demonstrated a far greater advance in performance over the 'V that gets the glory for the high rate of Fighter Command losses in 1941/42, although it was the Friedrich that was actually responsible for the majority of losses over France in that time.