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:
Roughly 18,000 men, including 4 infantry brigades plus other units like engineers, medical personnel, artillerymen, signallers, HQ staff, etc)
Roughly 4,000 men, in 4 infantry battalions plus other units
Roughly 1,000 men, in 3 companies plus other units
Roughly 230 men, in 4 platoons plus other units
Roughly 50 men
This changed over time - the cavalry was drastically reduced, stokes mortar teams were added, more machine gun teams were created, etc.