Before WW1 the army was optimised primarily as a colonial police force, coupled with a small expeditionary force of regular soldiers intended to deploy to the continent to work alongside the French or other allies in the event of war. WW1 was an event that really constituted three armies – the small regular/territorial force of barely 300,000 soldiers that mobilised in 1914 and was wiped out to buy time. The interim force of Territorials and Reservists that held the line in 1915-1916 while the army reconstituted, and the civilian volunteer/conscript force from 1916 onwards that saw the army grow to over 4 million men by 1918.
Rapid demobilisation followed, followed by regeneration in the 1920s and 30s to become the most mechanised army in the world by 1939, comprising some 224,000 regulars. It is often forgotten that the British army of 1940 had many more tanks and vehicles than the German Army – history is not kind to the losers. The army in WW2 grew to a citizen force of roughly 3.5 million men, before shrinking post war.
The continuation of National Service, the war in Korea and the end of empire saw the army stay at roughly 330,000 soldiers for much of the 1950s, causing significant damage to the national economy due to the cost and lack of manpower for rebuilding. By 1957 the army estimated that its regular strength was roughly 80,000 personnel (only a quarter of the whole force), many of whom were tied up training two-year National Servicemen. A major factor in the 1957 Sandys Defence White Paper was the need to reduce manpower costs and free people up for other economically important tasks.
The Sandys Review led to a reduction to 165,000 troops most of whom were focused on either colonial policing actions (it is often forgotten that in the early 1960s there were over 100,000 UK service personnel in the Far East) or deployed in Germany as part of the British Army of the Rhine (BAOR). The withdrawal from empire saw the army shrink to a strength of approximately 150,000 by the 1980s, where its role was primarily to provide a corps of four divisions in Germany in the event of general war, supported by mobilisation units from the UK which would provide further divisions to augment BAOR and conduct home defence roles.
The end of the Cold War saw the first deployment of a divisional-sized force, with an armoured division sent to the Gulf in 1990 for Operation Desert Storm. This happened just as the Options for Change review cut BAOR and reduced the army to approximately 120,000. Further deployments to Bosnia and Kosovo in the 1990s followed by the deployment of an armoured division to Iraq in 2003. Operations in Iraq and Afghanistan from 2003-2014 saw the army struggle to sustain itself on two fronts without heavy support from the RN and RAF providing extra manpower and resources.
The 2010 SDSR initially preserved the army at just under 100,000 personnel, although later reviews cut this down to 82,000 regulars supported by a target of approximately 30,000 reservists working in a far more integrated manner. Today the army is struggling to sustain itself at 82,000, with recent manpower figures showing a total of roughly 78,000 troops.
Sir Humphrey, “How Do You Solve a Problem Like a Deployable Division?”, Thin Pinstriped Line, 2017-08-06.
Private William Penman, Scots Guards, died 16 May, 1915 at 
