How did the British Army unit size and structure in WWI change from 1915 to 1918?

Question

Preface: For obvious reasons, we will assume ideal conditions for the purposes of this question. For convenience's sake, we will also ignore the original British Expeditionary Force that was almost wiped out by early 1915, and instead, limit the scope of the question to the "New Army" (AKA "Kitchener's Army") that took to the field from mid- 1915 onwards. And I know that some units were unique, sometimes drastically different from the norm, so answers will, of necessity, be general and not universally applicable.

I'm curious as to how the size and structure of British Army changed from 1915 to 1918. As I understand it, in 1914 the theoretical ideal was as follows:

Division:

Roughly 18,000 men, including 4 infantry brigades plus other units like engineers, medical personnel, artillerymen, signallers, HQ staff, etc)

Brigade:

Roughly 4,000 men, in 4 infantry battalions plus other units

Battalion:

Roughly 1,000 men, in 3 companies plus other units

Company:

Roughly 230 men, in 4 platoons plus other units

Platoon:

Roughly 50 men

This changed over time - the cavalry was drastically reduced, stokes mortar teams were added, more machine gun teams were created, etc.

Starting in 1915, excluding special cases, and assuming ideal conditions, how did these numbers change - including the number of men per unit (e.g., ~1,000 per battalion) and the number of subsidiary units per larger formation (e.g., 4 battalions per brigade)?

Answer 1

• The British introduced conscription in 1916, prior to that enlistees could choose which regiment or corps to join. Certain Regiments could trace their history back hundreds of years and typically drew from geographic parts of the country. After conscription in 1916 they were placed where they were needed not where they chose to serve.

• In 1914 an infantry brigade contained 4 battalions. In 1918 due to the inability of the army to replace heavy losses the number of battalions was reduced to 3 for 1 infantry brigade.

• The structure of a division varied throughout the war and its strength could fluctuate drastically if it had been heavily engaged.

source

Answer 2

Appendices A and B of The Official History of the Canadian Army in the First World War show that the divisions of the Canadian Corps were composed of three infantry brigades, plus divisional artillery and headquarters sections, throughout their deployment in France. In turn, these brigades were each composed of 4 battalions throughout their deployments.

The Canadian Corps was organized in a manner consistent with practice throughout the remainder of the British Army, including battalion structure at an establishment of 1007 men including 30 officers.

Substantive changes through the period of the war seem to have been solely in the allocation of support units such as artillery and other heavy weapons such as machine guns sections.