Military bugle call signals seem to have initially “evolved” ... but then been adapted into formal compositions and become standardized. Standardization, in the British Army at least, was complete by the early 19th century.

[1] EVOLUTION

From references in the Old Testament, we know that Horns (distant ancestors of Bugles and Trumpets) were used in ceremonies and also in battle. Examples: -

•Moses is told by God to “Make two silver trumpets; of hammered work you shall make them; and you shall use them for summoning the congregation, and for breaking camp." (Book of Numbers 10.1-10)

•Joshua and the Walls of Jericho. (Joshua 6.4)

•Gideon and the Midianites. (Judges 7.16)

Signal musicians (“aenatores”) as an integral, standard part of a military organization appear first in the Roman Legions. A collection of forty-three signals were used in the Roman Army.

In post-Classical history, the first reliable record of horns being used to issue battle commands was by the French army at the Battle of Bouvines (1214), in which king Philip Augustus defeated Otto IV of Germany.

[2] COMPOSITION AND STANDARDIZATION

In 1544, there is a reference to an English army specifying common bugle signals, to avoid confusion of meaning between units using different bugle calls for the same purpose. At this same time, it seems that bugles were being used for cavalry signals, while drums were being used for infantry signals.

In 1614, Cesare Bendinelli, and Italian musician employed by the Duke of Bavaria, published “The Entire Art of Trumpet Playing”, which included directions for playing the following military signals: -
"Bring up the Saddle"
"To Horse"
"Parade"
"To the Standard"
"Call to the Skirmish"
"Pitch Tents"
"To Retreat"
"To Watch"

From the above notes about the development of military bugle calls, it seems apparent that a system of at least semi-standardized calls was in place by the time that a standing British Army was established in the 17th century. I think that it is reasonable to presume that the various Regiments of the British Army shared many common signals right from the start ... although each early Regiment may also have employed unique calls to summon its troops to the Colors.

However, there are references to the British Army standardizing its bugle calls during the reign of king George III. This may relate to a 1799 publication, John Hyde’s “New and Compleat Preceptor for the Trumpet and Bugle Horn”. Hyde’s compilation lists Cavalry calls for Stable, Boots and Saddles, To Horse, and Water Call; and also some Light Infantry calls.

[3] THE LAST POST

I agree with your comment about the unusual complexity of “The Last Post”. There are (understandably, since the American Army adapted its bugle calls from those of the British Army) similarities between the American and British versions. But, for me at least, the British “Last Post” is the most emotionally evocative piece in any military repertoire. Whenever I hear it, I still stand to attention; I shiver; and (not ashamed to admit it) my eyes tear a little.

There doesn’t seem to be any absolute certainty about the composition of “The Last Post”. Several sources point to Franz Joseph Haydn (1732-1809) as the composer. Although Austrian by birth, Haydn worked in London during the reign of George III. So the connection between the standardization of British Army bugle calls under George III and a Haydn origin for today’s “Last Post” music may be real.