What if the Germans discovered chaff (reflective strips to spoof radar) early enough to use it in the BOB? Both sides discovered the technique in 1942, and both sat on it for a while because they were afraid it would be used against them. The idea was tossed around in Britain as early as 1937. So, what if the Germans figured it out early enough to use it against the Brits in the summer of 1940?
To what extent would the chaff reduce the effectiveness of radar? What countermeasures would the Brits be able to come up with? How much would any reduction in radar's effectiveness impact the BOB?