The United Kingdom and the United States are allies and have been for over 100 years. Between the North Atlantic Treaty Organization (NATO) and simply being on the same side in both World Wars, these two great nations have a considerable amount of shared struggle against common enemies.
They also enjoy the same language (though some might raise an eyebrow at that characterization) and a shared colonial history going back 400 years. They have had two wars against each other, but those were a long time in the past, and all is forgiven.
A portion of the modern United States' legal setup is borrowed from the UK, and certain ideas about democracy and equality likewise. However, the Enlightenment happened in both places at once, and the US has other influences such as France, Spain, and the Native American residents who were here before it existed.
That means differences pop up that even the most experienced pond-hopping traveler might not notice. These are often benign, like calling the same food a fry or a chip. Sometimes, they are rather embarrassing, for example, in the UK, an eraser is called a rubber (yes, really). Sometimes they're incredibly confusing, like writing the dates differently. Parsing out these differences is the sign of a true Atlanticist. Let's see if that's you!