I think that the first coins were a wee bit before king David1 for during the time of Celts there was coinage even then.
In Britain and elsewhere, a number of Celtic tribes issues coins in gold, silver, potin, and base metals. The Celts were here before the Romans, some became assimilated into Roman civilisation, others moved or were defeated. It is often stated that they left no written records, but this is inaccurate as some of their coins were inscribed and would bear the inscription "CUNO" an abbreviation for Cunobelin, a chieftain of the Catuvellauni tribe. It constitutes a written record, as do all the other inscribed Celtic coins, however most Celtic coins are unfortunately uninscribed. They also clearly show that the Celts used written language, even if other evidence is not existant.