This is pure speculation but we know now all the playable factions and the first culture pack faction for Attila. If we are to use Rome 2 release as reference there will be one more free faction released together (which was Pontus on released) and some more factions with the released of subsequent DLCS.
So for Attila, in case CA is following this method, what will the faction be ? My guess it will be one of the faction in Britain, possibly the Picts. It will be strange if CA did not include a playable faction from Britain.