I'll be in my bunk

as you can see from gluteus' comment
paper organisation and tactical use of troops are totally different things
military organization of tactical units is based on a theoretical concept of how independent units (batallion) are to operate and what elements they should contain
but also on logistical issues (tents, kitchen, bagagge train etc) and commanding elements needed in the actual period
this works well for a unit which is kept in barracks - and tactical organisation is about that matter
on campaign, the skilled officer (as Wolseley above) would sudivide the batallion as tactically necessary, also detaching troops from other units for support, adding logistical elements etc
unless You are gaming a historical scenario, where the actual composition is reported (as for isandlwhana above), you are totally free to include whatever You like, and from any units you like
what You can do is find out how british troops were housed during campaign and how many soldiers a cook and his aides would feed, as well as how many soldiers a baggage wagon could supply.
theoretically, this would be the minimum subunit to be detached independently, because this would be also a group of soldiers accustomed to each other, living together etc, and thus having been drilled together and cooperating smoothly
for example at Isandlwhana the companies are said to have formed firing lines on their own but it is hardly imaginable that the shooting drill would have been exercised in company strength
also when You see bodies of troops marching or parading on a daily basis, the company is subdivided -
only on special occasions do companies or batallions parade as one body, and this has to be exercised (watch "trooping the colours" on Youtube)
nowadays one barracks is housing one platoon, and this is the subunit for the daily routine
I could imagine that the formal platoon subdivion was established after campaign experiences, so Wolsley's orders make totally sense
however, if You want to wargame this, you will never form units bigger than 15, since this is unwieldy
unless of course You are attacking over open terrain against MG fortifications
this would be my approach
from my experience, two units of ca. 12 soldiers with firing drill rules (the british professional soldier -the old contemptibles- were so good at this that german units first facing them in 1914 mistook the drill for MG fire!) would be enough to hold at least 50 warriors who don't shoot back at bay, in any rules system (my experience includes T&T,Sword and the Flame, WHAB), and this without gatling guns or MGs
any regular british unit in colonial context (or european unit for that matter) would be able to massacre hundreds of native warriors unless outwitted or badly led, and make a huge fruit salad afterwards
