How to fight impetuous troops depends on what they are and what you have.
In the case of using Romans to fight Ancient British, I think the historical tactics are pretty good - sit the legionaries uphill and break the warbands' charge with the auxiliaries. Also, spread some Art (F) behind your line - kills are good, but even just pushing every second or third element back would be useful. If you do it right, you should be able to make first contact with overlaps, and that should knock a few holes in his line. Being uphill will also give you a +1 in his bound. Remember, Bd get a QK on Wb in the Bd's bound.
Alternatively, take advantage of the fact that you're likely to be invader, and therefore very likely to deploy second. He won't be able to change direction much, so you may be able to concentrate most of your army against one or two commands of his, and those facing air will take a long time to get to the battle.