This book should have worked. Parts of it are excellent, but there are significant editing and pacing problems. The first third of the book is repetitive. Several characters' backstories are told in flashback, then retold in dialogue, and sometimes mentioned again throughout the book. I sorely wished that I had had a red pen that I could have slashed out whole paragraphs to speed the story up. This book clocks in at over 600 pages. Vonnegut would have had a fit about wasting readers' time. Not only are backstories repeated, but history is given to us almost in lecture form by several characters. I understand why Sansom did this. His alternate Britain—bullied by Blackshirts and being dictated to by Germany—takes some getting used to. But it disrupts the illusion of realism. Would people who had lived under this since 1940 really recount (more than once) it all like a history professor?
Once Sansom starts to speed things up (around page 200), things get very exciting. At the heart of this book is a secret. That secret, leaked during a bout of confessional drunkenness by an American scientist to his British brother, might not have sparked an international incident were not for the fact that it was the secret of the atomic bomb. Frank Muncaster, a shy, reclusive astronomer, is horrified by his brother's revelation and accidentally pushes him out the window, ranting about the end of the world. He's sent to a mental hospital and his brother is arrested by the Americans. The British Resistance and the German Gestapo put two and two together and work out that Muncaster knows something. The Resistance dispatch civil servant David Fitzgerald and the Germans send Gunther Hoth to try and capture the scientist. Meanwhile, Fitzgerald is hiding his secret from his pacifist wife and the British Special Branch and pretty much everyone else in the British government.
The characters chase each other from Yorkshire to the Sussex coast, escaping just at the last moment and turning up where least expected. The middle 200 or so pages of this book are excellent, but things slow down again towards the end when Sansom has his characters remind each other of their pasts a few more times before trying to wind things up for the big finish.
The ending is, frankly, flat and anticlimactic. There is a big fight, but it's not at all satisfying. In the epilogue, Sansom returns us to where we started: in a room with Churchill and other politicians. I won't reveal the ending and ruin things for anyone who wants to read the book. I'll just say, try not to get your hopes up too much. I'm surprised that the other reviews I've read of Dominion don't mention the many problems. The other reviewers must have enjoyed the middle so much that they forgive the rest.