I have read several of if Barry Cunliffe's previous books, and particularly enjoyed the scope and breadth of "Europe Between the Oceans" (Europe Between the Oceans: 9000 BC-AD 1000), a work covering the same approximate time-period as this book, but a far larger area.
Britain Begins is even better than that due to its focus on a much smaller geographic area.
As would be expected from its author, this book really shines in the prehistoric era and brings together a wealth of modern thought onto the ancient evidence with an abundance of colour pictorial diagrams, maps and photographs. Often with prehistory certain knowledge is denied us; the book does a good job of asserting the evidence and then suggesting the most likely cause, but leaving open the possibility of error. If you care about ancient history or archeology you will certainly speed through this book, as for people with this kind of interest it is a real page-turner!
Not everything is perfect: some of the maps are coloured incorrectly, and after the brilliance of the pre-historic sections the saxon period feels rather threadbare (those wishing to learn more about that time should get The Anglo-Saxon World). But as an easy-reading introduction to the British past it is almost perfect and it seems churlish to deny it full marks.