Most of the coins come from the 1st Century BC or just before the Claudian invasion. There are no British coins dated to as early as 272 BC, although there were Gaulish coins being struck at this time (copies of the Massilian drachma with the head of Artemis and the lion, which is copied on some British coins also)

I'm afraid I seem to have hit a dead end, now. I don't think it's possible to pin down the Casse any more precisely. They seem to have been a sub-tribe either of the Catuvellauni or the Trinovantes, but otherwise nothing to do with Cassivellaunus. Either way, they would start very small in EB.