Preparations began after the Fall of France, when the Germans felt they had already won the war. Britain, however, refused to start peace talks, so more direct measures of reducing British resistance were thought of.
The Oberkommando des Heeres (OKH) planned a invasion of nine divisions by sea and two divisions by air. The chosen invasion site was along the coast from Dover to Portsmouth.
The Battle of Britain was originally intended to allow the Luftwaffe to achieve air superiority over the Royal Air Force and allow the invasion fleet to cross the English Channel. However, later on the Blitz instead became a strategic bombing operation. The transports to be used would be Rhine barges.
Most current military analysts do not believe that Operation Sealion would have succeeded if undertaken. The main difficulty was the lack of German naval assets in comparison to those of the Royal Navy.
In wargames conducted at the Royal Military Academy Sandhurst after the war, which assumed that the Germans had total air superiority, the Germans were able to establish a beachhead in England by using a minefield screen in the English Channel. However after a few days, the Royal Navy was able to cut off supply to the troops in England, and troops were then isolated and forced to surrender.
A mass invasion by sea however, may not have been necessary. In British wartime cabinet documents released in 1998, revealed that after the failure of the British Expeditionary Force in France and its evacuation at Dunkirk, Winston Churchill had lost support in cabinet and parliament. Had the Royal Air Force been defeated by the Luftwaffe, Churchill would have been replaced as Prime Minister by Foreign Secretary Lord Halifax, who was known to be in favour of peace negotiations with Germany rather than face a civilian bloodbath on British soil.
External link: British Invasion Defences