At the start of WW1, in 1914, the British Expeditionary Force (BEF) that fought in Europe was composed of six divisions of all arms (infantry, artillery, engineer) and one cavalry divisions; seven altogether. Each of the six divisions was made up of three infantry brigades, and each one of these brigades was comprised four battalions, totaling twelve battalions, with divisional mounted troops, signal service, supply and transport train, and field ambulance.
Each division of the BEF, which fought at the beginning of World War I, was composed of 18,000 men of all ranks, of whom 12,000 were infantry soldiers, equipped with 24 machine guns, and 4,000 artillery men, fitted out with 76 artillery pieces of all calibers (fifty four 18-pdrs, eighteen 4.5-in howitzers, and four 60-pdrs). The cavalry division included four brigades of three regiments each, and cavalry divisional troops. They totaled 9,000 men of all ranks, and 10,000 horses.
All these troops had been very well trained before being shipped to the continent. They were also very well equipped and armed, with machine guns, grenades, heavy guns and howitzers. They left for France under the command of Field Marshal Sir John French, who would be replaced by General Henry Kitchener in 1915, leading what was called then the Kitchener's New Army.
To summarize, at the beginning of the Great War, the British Expeditionary Force was roughly composed of 140,000 men, but by November 11, 1918, when the conflict ended, more than 2 million British soldiers had served and fought on the Western Front. By then, the BEF was composed of five armies: the First Army, under Henry Horne; the Second Army, led by Herbert Plumer; the Third Army, conducted by General Julian Bing; the Fourth Army, under the command of Henry Rawlinson; and the Fifth Army, commanded by General Hubert Gough. The Fifth Army would be wiped out by the German Spring Offensive of March 1918.
Below, the British Expeditionary Force in Action (footage)