I mucked around with the Egyptian chariots and those seem quite good, although they are NOWHERE near cavalry in terms of crashing into the rear of an enemy formation.

In the case of the Britons, i would just use the general's chariot to knock down infantry in (very) thin formations before my infantry/cavalry fall upon them, since that would make said infantry easy prey for any supporting troops. But you need to be careful since a formation thats too thick would bog your general's unit down, which is suicidal, or at the very least, get some of your chariots stuck and then killed.