I've read that the Romans took at least one elephant to Britain (probably the most intelligent thing in either army) - it outflanked a Celtic defensive position on a river by swimming across it (the river, not the Celts)..

What do people suggest one uses for early Imperial heffalumps?

Does it specifically have to be amphibious ?

I would have thought AMA 16, Successor elephant, would be the best bet.

Igor - I took that as a given, seeing as how they are factory-fitted with snorkelling gear.

Any Roman elephant - if any crossed the channel - would most likely have been North African and so models using the Numidian or Carthagian elephants would probably be most appropriate. There is apparently a painting depicting the scene in the Museum of London. The reference for your crossing is from Polyanenus (Strategms 8.23.5) who is describing Caesar's crossing of the Thames. Caesar himself does not mention elephants just mounted troops. There is some folklore about Claudius riding an elephant into Colchester, but this is based on extrapolation from Dio (LX.2.1). There is no evidence that I'm aware of for using elephants, post J Caesar, in a Roman army unless of course it is in Essex.