Britain’s mistake was not doing a Brexit in 1914. Why did she care so much for the Continent, when she had her Empire. Where did this sudden love for France come from anyway, after they had been enemies for centuries.
The French Ambassador asked Britain to join France and was told no. He was astonished and told the British Cabinet that the Foreign Secretary had promised British support, and as a consequence, France had moved all its Atlantic fleet to the Med for action against the Austro-Hungarian Empire, leaving France's Atlantic ports open to German naval attack.
This was the first time the Cabinet had heard of the Foreign Secretary's promises.
So, as a matter of honor, the British Cabinet voted to declare war the following morning.