1.
Preserve France as a Great Power
Grey saw in the Kaiser a new sort of Napoleon intending to subjugate Europe. He believed in the domino theory: first France, then Holland, then Denmark. Buchanan judges that 'Grey was tragically mistaken'.
2.
British Honor
The will to live up to the obligations towards Belgium. Had the Germans not invaded Belgium, had the Belgians not fought, the Cabinet would not have supported the ultimatum.
3.
Retention of Power
Basically career opportunism. The sitting cabinet feared it would loose the elections if the Cabinet would fall over the war and that Unionists would be brought to power who supported a war. They dreaded opposition. They went to war partly to keep the Tories out.
4.
Germanophobia (to be recognized everywhere where English comment on anything German even today)
Britain resented the rise of Germany and feared that a defeat of France would men German preeminence in Europe and the eclipse of Britain as an economic and world power. [911I: this happened eventually big time anyway, even after the destruction of Versailles and 1945-1990, with the UK probably turning into a pathetic Islamic Republic within 20 years. Bright light: the British are not racists! Idiots maybe, but not racists!]
5.
Imperial Ambition and Opportunism
The British war party saw France and Russia as bearing the cost in blood of land battle in europe while the Royal Navy, supreme at sea, ravaged Germany's trade, seized her markets, and sank the High Seas Fleet, as the empire gobbled up every German colony from Togoland to the Bismarck Archipelago [911I: hooliganism
avant la lettre wearing a top hat]. A war where France and Russia fought the German army, while Britain did most of her fighting outside Europe or at sea, matched perfectly the ambitions and strengths of the British Empire.