Mobile missile platforms
'They begin by driving all over the field and hurling their javelins. Then they jump down from the chariots and engage on foot. In the meantime their charioteers retire a short distance from the battle and place their chariots in such a position that their masters, if hard pressed by numbers, have an easy means of retreat to their own lines.' Gallic Wars IV: 1 Julius Caesar
So, it seems British chariots were used both as mobile missile platforms and as a form of battlefield taxi for high-status warriors. We cannot be certain that this description of their first-century BC battlefield application would apply to how they were used in the fourth century BC, but it seems possible that what Caesar witnessed was the continuation of a long-standing martial tradition.