1700ad is a good cutoff for the modern definition of Britain, which is when the island was unified under a single government. Norman Conquest was pretty transformative I heard, and by then the conquerors had already been assimilated, but idk how much that really matters since William wasn't like Tamerlane.

Caesar saw Europe differently back then but surprisingly not that different by geography, "omnia Gallia" divided by rivers plus Germany, Britain, and Spain. https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcT-R1QU...