The Battle for England (Ethelred Book 2)

September 1940: Can Britain see off the Nazi invasion? In the British command bunker, deep underground in a London suburb, Churchill and his commanders await the onslaught. They have lured Hitler into launching the attack before his forces are ready, pretending the RAF is defeated, and the Royal Navy on the edge of mutiny. Now the audacious plan to inflict a great defeat on Hitler is about to be put to the test.
Churchill is confident they can defeat the enemy. He doesn't realize that not all his enemies are abroad.