John O'Farrell's compendium on Britain's history basically covers the time between Julius Caesar's unsuccessful attempt to conquer the small island north of Gaul (55-54 BC) and the end of the Allies struggle with the descendents of these Gauls (1945 AD). The book is an accumulation of small sections, each dealing with one aspect like "Hadrian's Wall", "Queen XX", "King XY" or "Napoleon vs. Nelson".
In other reviews (see Amazon.co.uk), people were complaining about the lack of depth of John O'Farrell's presentation. The book tries to cover all important aspects of British history in 2000 years on 550 pages. As comparison: Gibbon's masterpiece on the fall of the Roman Empire covers about 1500 years on something like 2500 pages, just focussing on the aspect of decline. Anyone complaining should have done the math before buying the book.
So obviously this book is not intended to provide a scientific in-depth understanding of the threads which connect various historical events. Instead, for everyone unfamiliar with British history (like me, whose high school education in Germany carefully neglected every part of British history which did not include Prussian-educated kings like Wilhelm III or Austrian dictators like You-Know-Who), the book sums up all the important events and names that probably belong to general education in Britain.
More importantly, John O'Farrell manages to comment on the various events with typical British humour, making the read incredibly delightful (Example: "Women were clearly biologically different to men, they said, which made them physically unsuited to voting. It leaves you wondering which part of their body the men used to vote with."). I regularly found myself having read about the double or triple amount of pages that I had planed to read before turning off the lights for the night.
In conclusion: This book is an easy read, which will provide you with an overview on British history, a serious of smiles and some laughter. If you're interested in the details below the surface, you should pick out other books which focus on specific topics.