Re: Will you kindly stop with the "Britain alone" myth?
Broadly I agree, but the Royal Navy wouldn't have been sunk in anywhere near such numbers.
'Defeat' for the RAF means that the fighters are pushed back north of the Thames. (The pilots were tired and numbers of pilots was a concern, but it was the damage to the airfields that was the critical factor).
At this distance from the south coast, the single engined German fighters can't reach to escort the bombers. When the Germans tried bombing raids from Norway (escorted by twin-engined long range fighters) the raids got shot to pieces, and those raids were quickly discontinued.
So the Germans can't destroy the RAF, since they can't hit the airfields in the midlands and the north.
The RAF fighters pushed back to north of the Thames still means full RAF air cover to the Royal Navy down the majority of the east coast. Any German air attacks on the navy in transit are coming form Norway, and will suffer largely the same fate as the historical bombing attacks.
With the RAF fighters operating from north of the Thames, they can still contest the air space over the south coast, so the German's can under no credible scenario achieve the level of air superiority the allies had over Normandy - and even in the 1944 Normandy landings, the Germans were able to put up some sorties.
During Dunkirk, the German sank about half a dozen destroyers through air attack. Here, the Allied fighter cover was flying from Britain, and the Germans were probably based closer to their target than the British air cover was. Also the ships were typically stationary or moving slowly close to shore. In an anti-invasion scenario, the RN's destroyers are coming through the Channel at speed and manoeuvring sharply: much harder to hit.
The effectiveness of air-power in the anti-shipping role achieved from mid-war onwards, particularly in the Pacific (generally with aeroplanes and weapons specifically designed for the task, with aircrew trained for the task) masks the much more limited effectiveness of air attack in anti-shipping operations achieved earlier in the war, when the attackers had been trained and equipped primarily to attack land targets.