Would Hitler have won World War II if he had left Russia alone?

[longship]

111. There was no way that Hitler invades Great Britain.

And Churchill knew that from day one.

In his great speech upon the rescue at Dunkirk, he addressed this directly.

Nevertheless, our thankfulness at the escape of our Army and so many men, whose loved ones have passed through an agonizing week, must not blind us to the fact that what has happened in France and Belgium is a colossal military disaster. The French Army has been weakened, the Belgian Army has been lost, a large part of those fortified lines upon which so much faith had been reposed is gone, many valuable mining districts and factories have passed into the enemy's possession, the whole of the Channel ports are in his hands, with all the tragic consequences that follow from that, and we must expect another blow to be struck almost immediately at us or at France. We are told that Herr Hitler has a plan for invading the British Isles. This has often been thought of before. When Napoleon lay at Boulogne for a year with his flat-bottomed boats and his Grand Army, he was told by someone. "There are bitter weeds in England." There are certainly a great many more of them since the British Expeditionary Force returned.

Of course, he later finishes the speech with a typical and iconic flourish.

Turning once again, and this time more generally, to the question of invasion, I would observe that there has never been a period in all these long centuries of which we boast when an absolute guarantee against invasion, still less against serious raids, could have been given to our people. In the days of Napoleon the same wind which would have carried his transports across the Channel might have driven away the blockading fleet. There was always the chance, and it is that chance which has excited and befooled the imaginations of many Continental tyrants. Many are the tales that are told. We are assured that novel methods will be adopted, and when we see the originality of malice, the ingenuity of aggression, which our enemy displays, we may certainly prepare ourselves for every kind of novel stratagem and every kind of brutal and treacherous maneuver. I think that no idea is so outlandish that it should not be considered and viewed with a searching, but at the same time, I hope, with a steady eye. We must never forget the solid assurances of sea power and those which belong to air power if it can be locally exercised.

I have, myself, full confidence that if all do their duty, if nothing is neglected, and if the best arrangements are made, as they are being made, we shall prove ourselves once again able to defend our Island home, to ride out the storm of war, and to outlive the menace of tyranny, if necessary for years, if necessary alone. At any rate, that is what we are going to try to do. That is the resolve of His Majesty's Government-every man of them. That is the will of Parliament and the nation. The British Empire and the French Republic, linked together in their cause and in their need, will defend to the death their native soil, aiding each other like good comrades to the utmost of their strength. Even though large tracts of Europe and many old and famous States have fallen or may fall into the grip of the Gestapo and all the odious apparatus of Nazi rule, we shall not flag or fail. We shall go on to the end, we shall fight in France, we shall fight on the seas and oceans, we shall fight with growing confidence and growing strength in the air, we shall defend our Island, whatever the cost may be, we shall fight on the beaches, we shall fight on the landing grounds, we shall fight in the fields and in the streets, we shall fight in the hills; we shall never surrender, and even if, which I do not for a moment believe, this Island or a large part of it were subjugated and starving, then our Empire beyond the seas, armed and guarded by the British Fleet, would carry on the struggle, until, in God's good time, the New World, with all its power and might, steps forth to the rescue and the liberation of the old.

Arguably Churchill's greatest wartime speech. You can read and/or listen to the whole thing here

Churchill knew that a German invasion was a fool's errand. The Brits certainly prepared for the eventuality of such a thing, but those high up in Britain's government knew it was not likely to happen. Germany just did not have the facilities to accomplish such a task, and Hitler himself wanted to ally with Britain, not conquer it. Churchill knew that, too.