First of all, are you playing strategically enough? If you can suck the enemy into attacking a force holding a bridge over and over, then the enemy won't have many armies left to fight with.

Secondly, have you tried using onagers? At least four per army help immensely, since the Britons can't build anything of that range.

Thirdly, again using onagers, try to attack as many of their cities as possible. Chariots are not as useful in cities. Use the onagers to break down sections of the walls, and your archers to lay down covering fire while your men enter. The Britons will take a lot of casualties from your missile fire. Slowly advance into the city with your archers providing covering fire. You should be able to squeeze them into the central square where all that being jammed together will stop a lot of the manouverablility of the chariots.

I also suggest as much micro management as possible for missile units. Whenever you see chariots, get your archers to fire on them. That way you can eliminate them as soon as possible in battle.