FWIW the V3 call works. So I don't know whether I was using V2 incorrectly, or it's just not supported.
For reference here's a working example:
restApi.getPricesV3(authenticationContext.getConversationContext(), "1", "4", "200", "IX.D.FTSE.CFD.IP", "2021-01-19", "2021-01-21", "DAY");