Ignoring "proto-money" the first objects that can be described as coins are the potins normally found in south-east England. These may perhaps have had more of a ritualistic signficance rather than being a medium of exchange.
Some unidentified Roman prototypes of British Celtic coins, David Woods, 2013
Iron-Age and Roman Coins, J. Williams, 2003
Other References : PAS Iron Age (Celtic) coin guide Britain's First Coins - Chris Rudd