I find it hard to believe that an historian I respect on a great many things can be so wrong in this. The state of war by the UK was made to the public by radio with this announcement:
“This morning the British Ambassador in Berlin handed the German Government a final note stating that, unless we heard from them by 11 o’clock that they were prepared at once to withdraw their troops from Poland, a state of war would exist between us.
I have to tell you now that no such undertaking has been received, and that consequently this country is at war with Germany.”
France declared war on similar terms that same afternoon.
Poland was the reason. If Britain had merely wanted an excuse, it would have declared war when Germany moved on Austria, or any of the other land grabs, or simply for breaking the Versailles treaty.
Britain honoured its commitment in far more than just words. The declaration of war cost many millions their lives, led to the destruction of many of its own cities, and put the nation into massive debt that took over 50 years to pay back to the USA.
Russia caused a problem for Britain wanting Poland to be free. But at the end of the war with Germany, the heavily-indebted Britain, faced with a commitment to deal with Japan and the remaining Axis allies, just didn’t have the power or the economy left to war with Russia, certainly not on the European continent.
Poland was ultimately the reason, not an excuse, for the outbreak of the 2nd World War. That the war ended without Poland being free was tragic.