I managed to grab five minutes today to hunt down the Dux Bellorum armies (not where I thought I'd left them!) and split them up into three. I also found a couple of packs of Old Glory Carolingians amongst them!
The Romano-British army looks like this:
Mounted Companion - 1 unit (3 miniatures)
Noble Riders - 3 units (3 miniatures per unit)
Ordinary Shieldwall - 3 units (8 miniatures per unit)
Bow - 1 unit (6 miniatures)
Army composition has been driven by what I had miniatures wise rather than any great tactical nous on my part.
I am short one miniature for the Mounted Companion base, one rider for a Noble Rider unit and one Shieldwall foot. Rather than get too anal about this I am thinking that I should not worry that the bases don't contain the exact number of figures as long as it is clear what they are.