First impression, on early English history: it’s all Vikings. The Roman Britons were pushed out by Germanic raiders who seemed pretty similar to the raiders later known as Vikings. This initial wave of settlers was the “Anglo-Saxons.” Later, other Viking raids and settling continued. England even had a Viking king, Cnut the Great, shortly before the Norman Conquest.
Second impression is a continuation of what I realized from reading earlier books – the world is in a constant state of change. Peoples are always on the move; no one is an “aborigine.”