No need to upgrade British chariots. Once you get used to them they will win every battle for you. Currently I fought all Roman factions. Usually I try to flank them with two heavy chariots and hit them in the flank or rear. In the meanwhile my ranging troops soften them and the infantry is passive. Firts eliminate all enemy cav. Heavy chariots work very well. Then attack the inf. They will retreat at once. You cann roll through theír lines from one side to the other. When they start routing let your infantry chase them.