I couldnt possibly be able to tell you if they come later but chariots have a role if you know how to use them.

The fear they inflict on enemy troops.
When attacked by enemy cavalry that you will face in Gaul they seem to inflict great causualties, even if they arent great fighters.

Also your fighting with what the best the British Isles could give 2300 odd years ago.
Wise up!!! The British tribes were not fielding heavy calvary at this time. This is not an organised "empire building" culture your playing with its Iron age farmers who live in what was then the very edge of the world. British and Irish horses at the time would be little more then ponies and such could not support a heavy armoured man on their backs.

This game is about Historical accuracy some unit types in the Casse Roster are already questionable and i think Heavy calavry unit would be Taking the Mik on a massive scale!