The Swastika
The Swastika, naturally, is symbolic of the Nazi party. When he founded the Nazi party, Adolf Hitler decided to coopt a Hindu symbol, turn it sideways, and make it the symbol of his party. Because of their actions over the course of their existence, the Swastika also became symbolic of genocide, the Holocaust, violence, death, and oppression.
Winston Churchill
As the leader of the country, Winston Churchill is symbolic of Great Britain's grit, determination, and desire for success despite insurmountable odds. It was in part because of the strength of Churchill that the British people were able to survive the war and succeed, with the help of its allies, against the Nazi war machine.
The Bombers
The bombers that are featured so prominently on the cover of the book is symbolic of the seemingly insurmountable odds the British people faced in the face of surefire destruction by the Nazis. They had to face a seemingly endless number of bombers, bombers which bombed them almost nightly.
London
The capital city of the United Kingdom, London is symbolic of the heart of the British empire and symbolic of the British people's strength. Despite the trouble and danger they faced, London and its people remained strong despite nightly bombings, a lack of food, and otherwise bad conditions.
The Grit and Determination of the British People
An incredibly common motif in Larson's book involves the grit and determination of the British People, who, despite bad conditions and the constant threat of death, survived (and in some cases, even prospered).