The earliest coins found in Iron Age Britain date from around the second century BC and, until recently, it was believed that they were produced in Gaul (a region roughly equivalent to modern day France and Belgium) and imported into south-east England. These coins, known as Gallo Belgic A Celtic. They were struck by hand in gold, silver and bronze and used for approximately 150 years (they were based on the gold coinage (staters) issued by King Philip II, ruler of the ancient Greek kingdom of Macedon from 359 – 336 BC and father of Alexander the Great). Coins were brought to Britain by trade, as pay for returning mercenaries, by raiders and settlers from the Continent, and gifts between tribal leaders.
References
https://www.vosper4coins.co.uk/history.htm
http://blog.britishmuseum.org/2014/04/11/the-die-that-struck-britains-first-coins/
http://www.wnccoins.com/0024.htm