The first folks who started to write things down about the island were the Romans. They saw, they came, and they conquered as the saying sort of goes. For some 400 years they gave it their best shot to take complete control of all those folks who were already here. Peter Salway does a excellent job on organizing and discussing this period of Celtic history. "The Oxford Illustrated History of Roman Britain" it is called.
Illustrated it is with over 30 color plates, and 10 detailed maps. Chronology, chronology, chronology is the greatest help for us chronology lovers. From the iron age warrior to the last days of the Empire, a detailed account is provided. Published 1993 by the Oxford University Press, it has proved a great resource and reference for my tree climbing.