Briton was rich in minerals and had surplus wool and even some surplus grain to trade with.
Trade across the sea became common and many Britons used the same language as others in Europe. We now call this the Celtic language after the tribes who lived in what is now France. But Britons also picked up ideas from the Romans, as we can see by looking at British Iron Age coins. The only difference was that Britons mainly used them as offerings to their gods, not usually as money.