Okay, I know that Nazi Germany can't pull off an invasion of the United Kingdom, that isn't the point. Just want to get this out of the way right now.
My question is this. What would be required for a country to be able to invade and occupy the United Kingdom? Which countries could've fulfilled the criteria for doing so? A Germany that focuses on the United Kingdom? Some bizarre union between France and Spain(how I don't know, just an example.) Of course, could the United States have?
Or is the United Kingdom some invincible country during the early 20th century that is impossible to invade for any country on Earth for either logistical, political, or what have you reasons?
So that is my question, and my challenge. The cut off date is 1944, it must happen before then.