With a POD after 1900, there are only two countries that might do a Sea Lion, Germany and the Soviet Union / Russia. The US has obviously a much bigger potential, but a war between the US and the UK is much more unlikely, and the US would also need a base in France or the Low Countries from which to launch the invasion, which makes it a bit more unlikely still.
With Germany being much closer geographically to the UK, it is more likely to have the opportunity to manage a Sea Lion than the Soviet Union / Russia, but it is still rather unlikely. One basic problem is that in its war plans Germany is much more concerned with its continental neighbors than with Britain, and that the German leadership is not likely to predict that it will ever be in the posession of the Channel ports, with France completely defeated.
Another one is the fact that a German government that is intelligent enough to be able to mobilize Germany's resources to the extent of being able to invade Britain, is very likely also intelligent enough not to start a war against Britain in the first place.
But then again I feel tempted to show that, while unlikely, it is not altogether ASB.....
Germany's industrial potential is somewhat bigger than that of the United Kingdom.
Between the fall of France and Hitler's attack on the Soviet Union, the British Empire was facing a German-dominated Europe alone. For Britain, there were two potential Allies, the USA and the Soviet Union. I think a scenario is possible, where both are not potential allies. If Germany starts the war with a surprise attack on France and never attacks Poland, Poland is very unlikely to attack Germany. After all, France and Britain did very little in practice to relieve German pressure on Poland, and Poland, as the weakest country of the alliance Britain - France - Poland is likely to do still less for Britain and France. If the German attack on France and the Low Countries is a "bolt out of the blue", the Western campaign might be decided even before Poland has fully mobilized. Let's also say that in this timeline, there are no German demands on Poland. Poland would never allow Soviet troops on its territory and it cannot wage war against Germany on its own, so it is in effect a buffer state between Germany and the Soviet Union.
To eliminate the USA as a potential ally for Britain, we need an isolationist US president, and I think Robert Taft is the one who is most often mentioned in this context. In order to strengthen isolationism to make Taft president in the first place, I can think of three things that Britain can do: defaulting on its World War One loans, stronger and earlier protectionism, and a more repressive policy in India. I think all three of these mistakes could be made by a sufficiently right-wing, Mosley-like figure as prime minister. In order to bring someone like that into power, Britain must be in a much bigger crisis after WW I than it was historically. The most obvious reason for such a crisis can be a World War One outcome that is much less favorable to Britain and its allies. Two of the three mistakes that alienate the majority of the US population, namely more protectionism and repression in India, are also factors that weaken the UK by themselves.
So in order to get a situation that is favorable to a Sea Lion, we need a WW I with a result that is much more favorable to the Central Powers.
To be continued