This is a well told revisionist version of how the Romans and the Saxons actually took over Britain. The Britons hated the tribe next door more than the overseas newcomers, and invited in the Romans and Saxons to help them in their local rivalries. The Romans were happy to perpetuate and rule through the tribes, and the defensive works of later Roman Britain reflect inter tribal violence, being mostly on tribal borders rather than against external marauders. After the Romans had gone, the tribal kingdoms reasserted their political independence, in a process the author usefully compares to the revival of local nationalisms in late 20th century eastern Europe and Africa. The tribal kings then called in the Saxons as hired swords and settled them on disputed tribal borders, and in due course the Saxons took over the tribal kingdoms more or less intact as the starting point for their own disunited kingdoms - the Heptarchy.
The argument is credible and persuasive. Both Caesar and Gildas commented on the Britons' disunion, and the author shows how the defensive and military archaeology matches the tribal geography. The Catuvellauni in particular made enemies of everyone around as they tried to push out from their territories north of the Thames. Boadicea's Iceni tribe burned London and St Albans in AD 61 not simply because they hated new towns but because they hated Catuvellauni towns. For the Romans the province was routinely treated as a PR opportunity or a launchpad to take over the rest of the empire. The Britons always thought in tribal terms - they had no idea a United Kingdom would come along over a thousand years later. The barbarians in other Roman provinces were strong enough to stop this sort of balkanisation when the western Empire fell, but the Saxons in Britain were not strong or united enough to do the same for another five hundred years.
So why only four stars. Well the author could have tried harder to fill in the historical gaps in the Roman period : how far did the Romans treat Britannia as an entity ; did the later Roman "sub provinces" match the tribal borders ; what was the impact of the large permanent Roman military presence upon the tribes ; did the local aristocracy view themselves as British ? What part in all this was played by the Christian church, and why did it vanish ? Also the author rightly complains of the poor historical record, so it is surprising that he feels able repeatedly to contradict or ignore Tacitus who is the best historian that we do have - his father in law had governed Britannia for eight years soon after Boadicea's revolt. Tacitus is clear and plausible for example on the causes of Boadicea's revolt (greedy and overbearing Romans), he does detail the British atrocities (including crucifixions - presumably picked up from the Romans) and he is also clear that it was Boadicea who attacked the Romans in the final battle (with or without a stirring speech first). However, these lapses do not undermine the book's central point.
Finally, as the author says, several of the tribal kingdoms survive to this day, as Sussex, Kent and Essex for instance. So when these counties clash at cricket, they are prolonging a two thousand year old struggle.