Well on one hand, moving further north creates a pretty strong pressure for early humans to evolve lighter skin, so naturally the first early humans to move nor

Well on one hand, moving further north creates a pretty strong pressure for early humans to evolve lighter skin, so naturally the first early humans to move northward to a place would on average have darker skin than their descendants down the line. Though presumably some of that would have happened before those early descendants got all the way to the northern shores of France and Spain to move to Britain, so even granting them some benefit of the doubt, this does seem a bit much.