Regular British army regiments in Gail's alternate reality were similar to those of the real Victorian era, only a regiment was usually a little smaller (600 instead of 750+), with 6 companies of 100 men each. Soldiers in this kind of regiment might also be referred to as Regulars.
A different style of regiment based more on the militia model also exists in Gail's universe, of which the only named one (so far) is the Coldsteam Guards. Soldiers in these regiments could be referred to as Irregulars. These regiments number 100-200 men of special training and skills, often (although not always) formed around and including a werewolf pack (or, in the case of Woolsey, PART of a pack). One might think of these as special forces, designed to work at night, comfortably alongside the supernatural, sometimes in secret. An Irregular company would be responsible for the care and cultivation of ghost contacts.
Irregulars can form up with their real world counterpart, if needed. Thus the Coldsteam Guards can become an adjunct attachment for major action with the Coldsteam Guards (Regular Infantry). Or the Afterlife Guards may join with the Life Guards (Regular Household Cavalry). The Afterlife Guards, based in London and rarely sent overseas, includes the Queen's private werewolf guards (the Growlers) and, occasionally, vampires.
"Professor Lyall himself held the rank of lieutenant colonel with the Coldsteam Guards, but had, in the last fifty years …" (Blameless, Chapter 7).