If you are thinking of buying this book, I would certainly recommend it as great value for money. It is lucid and packed with interesting facts about every era of British history, so you are bound to have your mind expanded in some way.
The author was Chief Archaeologist of English Heritage and brings a huge amount of personal knowledge and experience to the subject - he seems to have done one or other excavation relevant to practically every subject he talks about, and to have spent time all over the British Isles.
The basic idea of the book is to start at the beginning and talk about the successive waves of people who have come to the British Isles - from the pre-H. sapiens Boxgrove man of 500,000 years ago, via the first modern humans arriving after the last glacial maximum, the Celts, the Romans, the Anglo-Saxons, the Vikings, the Normans, the Huguenots, the East European Jews, the West Indians who came on the Windrush in 1948, and the Ugandan Asians, up to the Somalis arriving as we speak.
The trouble is that the author continually loses focus and the book degenerates into a (very readable and original) social history of Britain. At one point, after reading several pages on the Vikings in Iceland, I thought "hang on, what has this got to do with the matter in hand?" The answer is, not a lot; the author just got carried away retailing his knowledge of the Viking migrations - but it was interesting all the same.
As far as the book's ostensible purpose is concerned - i.e. the ethnic make-up of the British population and how it got to be that way - it all ends up being rather vague. This is no doubt a reflection of the fact that no one really knows to what extent, say, Anglo-Saxon invaders displaced an existing British population; people argue about it but there are no definitive answers. The author makes some passing references to DNA studies, but he does not much use them in his main narrative. On the other hand, those who do use the DNA to build a picture of the biological origins of the British population (e.g. Stephen Oppenheimer) can seem to be missing the point when they imply that their forensic accounts of ancient migrations explain who we are and render conventional history and archaeology obsolete. Surely, it is things like the Roman period, the Norman invasion and the arrival of the Huguenots that are truly relevant to understanding modern British society, not what we've got on our Y-chromosomes. It doesn't matter whether I'm biologically descended from the Normans or not - the way I live is still shaped by their legacy. In this respect, the present author, by focusing on the social effects of the migrations rather than on numbers and percentages, can be said to supply a good antidote to the "DNA fundamentalists".
For me the most annoying aspect of the book is the underlying "right on" attitude. This is something it is hard to put your finger on, but there is a subliminal tendency towards looking down on the people of the past who did not share our modern concerns for equality and human rights regardless of class, gender, race or sexual orientation. It sometimes seems that the author's basic thesis is that the British are and always have been racists. On the one hand, he describes how Britain has repeatedly accepted refugees and taken a principled stance over things like the slave trade, but on the other he also makes sure to mention lynchings and rabid rhetoric against blacks or Irish Catholics etc., sometimes quoting extremists as if they represented mainstream opinion. You could say this is balanced but the scales always seem to tip slightly towards representing the British as peculiarly hostile to outsiders. In other words, there is a bit of a guilt trip involved. To give an example, with reference to Britain's Aliens Act of 1905, which restricted immigration for the first time, we are told the Tory government "succumbed to...pressure" and "xenophobia was made respectable" - a rather loaded statement betraying little sympathy for the concerns (misguided or not) that lay behind the act. By contrast, we are told simply that there were "restrictions on entry to the United States as a result of the McCarron Act [sic] of 1952"; i.e. when it comes to the US we get a neutral statement with no mention of anyone succumbing to xenophobic pressures (pp. 429, 441).
To finish on a positive note, one thing I liked about the book was the author's eye for detail. When he mentions the Huguenots, for example, he takes the trouble to explain where the word comes from (actually two competing theories), and this is typical throughout. He explains why and when ideas, names and practices arose so that dimly remembered factoids from one's schooldays begin to slot into place and make sense.
Overall, it's not quite what it says on the tin, but it remains an interesting perspective on the history of the British Isles.