So his army consists of 3BG's of light chariots, 2BG of light horse, 4BG of Warriors (all 8 bases each), 2BG of Elite Warriors (8 bases each - eek!), 3 BG of slingers (6BG each) and a fortified camp.
So how to combat this horde? Well the Roman army I've come up with consists of no less than 6 BG of Auxiliary Foot and 5BG of Legionaires (with attached Bow) and 2BG of Symmachiarii (medium impact foot). For cavalry I'll have 1BG of Auxiliary cavalry and 2BG of equites sagitarii. Not enough to even dent the massed chariots but enough to take on the Britons' light horse. The foot will have to deal with the chariots as best they can.
So that's 13BG of foot to face off against 9BG of foot / light foot. The weakness appears to be the lack of skirmishing - but note that the legionaires come with attached bow (6" range). Slingers will have to come within bow shot range to get their shots in (against armoured opponents which pretty much negates them) and the danger of being charged and caught by the Symmachiarii (who as impact foot will eat slingers for breakfast)
The big danger is lack of cavalry. But if we can get to grips with the infantry quick enough then it shouldn't matter. I'll have armoured, drilled troops all down the line to combat the larger numbers and as long as I can get the Superior Legionaires into the Elite warriors then the armour and skilled swordsman status will more than overcome the difference in class. I hope.