In BKC terms the "formation" is a smaller group under one HQ stand. Rigid doctrine HQs can't command stands from other formations at all, normal can at a penalty, flexible command anyone with anything anytime.
BKC probably works out at a large game being a regiment (or Commonwealth Brigade), I guess? Bigger than that and you'll not finish a game in less than a weekend! Your CO stand would be the Regt. HQ, then a HQ stand for each battalion.
Unless they're very small battalions you'll want at least one more HQ stand for each - they might represent the 2iC or XO or senior/most reliable Company commander, whoever drew short straw and got the "forward detachment"/probing force/dumped with defending the main terrain etc etc.
HQs are not necessarily "actual officer of rank X and his radio" but more a "focus of your command & logistics in that place and day", so don't worry about exactly replicating specific real world headquarters elements, just the overall level and focus of command you find you a/ need in game terms, and b/ reflects your interpretation of the real world flexibility and control!
That might change a little with the new 3rd ed as command stands can now be shot at, but the theories the same