Taking up Danny,s call to read the book. Take a look at a copy of Richard Cox's Operation Sealion, if you can find one. This largely details what was likely to have happened had Hitler launched Sealion in September '40. According to the war gaming that took place at the Royal Military Academy, Sandhurst after the War the Germans would have lost after a seriously bloody battle. The six referees, German and British were all in agreement with that result.
Yes we very nearly lost the Battle of the Atlantic but as Danny says thanks to Ultra and a lot of very brave Naval and Air Force Allies we won.
One battle we also very nearly lost was the Blitz but thanks to Hitler turning his attention East we just about got through. A fairly recent estimate suggests that in todays money the Luftwaffe caused over five trillion pounds worth of damage to our infrastructure and industry in eight months of night bombing. With civil insurrection looming ever closer by March 1941, Churchill could not have felt very comfortable.