One of the most deeply rooted myths of World War Two is that victory in the Battle of Britain prevented a German invasion of the United Kingdom. The reality is that the strength of the Royal Navy's Home Fleet prevented invasion in 1940. The Royal Navy was then a massive fighting force; consisting of 8 aircraft carriers, 12 battleships, 50 cruisers, 94 destroyers, 87 escort ships and 38 submarines. Germany's Kriegsmarine was puny by comparison, consisting of 5 battleships, 6 cruisers, 17 destroyers and 57 submarines at the start of the war in September 1939. By August 1940 the Kriegsmarine's strength had been further reduced by loss of the Graf Spee, plus the losses of the Norwegian campaign. In reality the Royal Navy would have smashed any attempted German invasion within hours of the ships leaving harbour, something Hitler was well aware of at the time.
The historical date on which England was genuinely spared from invasion was 24'th May 1940; the day Hitler gave Von Rundstedt the now infamous 'Halte' order; which both forbade him to employ the panzer columns against the British troops trapped on the Dunkirk beaches, and also forbid any German unit to move within 10 kilometres of the Dunkirk positions. This decision is now believed by most historians to have cost Hitler the war; and is frequently used to illustrate Hitler's military incompetence in modernday history books/dvd's etc...
Ironically; the truth is that Hitler, horrified at the already high casualty lists, and believing those same British soldiers would soon be marching alongside his own into Soviet Russia; issued this order to prevent even more 'pointless bloodshed' between what he regarded as kindred Aryan races. Had Hitler accepted the less racist but more ruthless advice of his General's; and continued the air/tank attacks on the beaten, disorganized and demoralized British troops within the Dunkirk salient; the famous 'Operation Dynamo' evacuation would have been impractical. The loss of the entire British Expeditionary Force would have been a monumental disaster which Churchill's Government could hardly have survived; while the political leverage offered by possibly 250,000 British POW's would have virtually guaranteed Sir Oswald Moseley as his sucessor. The resultant united Nazi Europe, plus the additional numbers of British troops/weapons; would in turn have made 'Operation Barbarossa' a far less hazardous undertaking.
An excellent documentary which covers both this and other Nazi mistakes is David Hoffmann's:-How Hitler Lost the War [DVD]  [Region 1] [US Import] [NTSC]