Yeah, I highly doubt Britannia is supposed to be a representation of America and their depiction as bad guys an attack on us at all. They just wanted the evil empire to have a sort of old school aristocratic/noble style or what-not, so they did the alternate history the way they did.
Anti-Japanese, probably not that either, I don't think. But still, it is kinda funny, all the hardcore Japanese nationalists are portrayed as incompetent, the Japanese resistance movement accomplishes jack fucking shit before a Brit comes along to lead them (even Toudou's 'miracle' was due to Lelouch trading military secrets to break off Nunnaly's engagement to Suzaku's dad,) hell, their best pilot is only half Japanese.
If anything I think the show is against people who judge people or place any importance whatsoever on things like nationality, race, etc. The people who fight only for things like their country (JLF) or race (Purists) virtually always lose to the people who fight for ideals(Suzaku) or people(Lulu).