The Brits did not have the benefit of knowing just how constrained the German military was, and all of their prior estimates had just fallen apart in France, so a bit of preparation was not, perhaps, entirely amiss.

https://en.wikipedia.org/wiki/Operation_Sea_Lion

As a precondition for the invasion of Britain, Hitler demanded both air and naval superiority over the English Channel and the proposed landing sites. The German forces achieved neither at any point of the war.

The Battle of Britain was about achieving that air superiority. At the time, both the British and Germans thought that Germany was closer to achieving this than was actually the case.

https://en.wikipedia.org/wiki/Battle_of_Britain#Attrition_statistics

Throughout the battle, the Germans greatly underestimated the size of the RAF and the scale of British aircraft production. Across the Channel, the Air Intelligence division of the Air Ministry consistently overestimated the size of the German air enemy and the productive capacity of the German aviation industry. As the battle was fought, both sides exaggerated the losses inflicted on the other by an equally large margin. The intelligence picture formed before the battle encouraged the Luftwaffe to believe that such losses pushed Fighter Command to the very edge of defeat, while the exaggerated picture of German air strength persuaded the RAF that the threat it faced was larger and more dangerous than was the case.[269] This led the British to the conclusion that another fortnight of attacks on airfields might force Fighter Command to withdraw their squadrons from the south of England. The German misconception, on the other hand, encouraged first complacency, then strategic misjudgement. The shift of targets from air bases to industry and communications was taken because it was assumed that Fighter Command was virtually eliminated.[270]