Why they did not nammed it the ''British'' Empire ?
Just for the record, it was probably given the Latin name because a) it is appropriate for the period (Latin was the Lingua Franca, any 'Empire' would by necessity be associated with Rome, etc), b) it isn't an obscure name for Britain, even if it is archaic, and c) well, Britannia sounds ten times more awesome than Britain. 'Britain' is a bit of a rubbish name for it, actually. It even rhymes with 'Briton', resulting in Americans and Canadians writing about the time they 'went to Britain to meet the Britains'.
Back on topic, I don't know for sure because I only got into Paradox about a year or so ago, but I suspect the anime world is probably quite obscure to the majority of Paradoxians. Definitely too obscure for an official DLC or game to be based on it, though maybe a mod would get a modest following.
EDIT: By a 'modest following', I mean a lot smaller than the Game of Thrones mod.