At the outbreak of World War I, Britain had a relatively small professional army (247,000 men). Close to half of them were stationed overseas throughout the British Empire.
Thus, on the home island in August 1914, Britain’s generals mustered about 150,000 men to be the British Expeditionary Force (BEF) that crossed the English Channel, to join the French in fighting the German attackers.
Within three months, that half of Britain’s professional army was gone. Most of the men in the BEF were dead.
p.s. Britain’s total WWI casualties: 673,375 dead and missing, 1,643,469 wounded
Howard Zinn, A People’s History of the United States, 1492- Present (New York: Harper Perennial Modern Classics, 2005), 360.