It is indeed Celtic Britain. The coin appears to be a "reduced weight type" debased-gold stater of the Corieltavi.
UKDFD match. It looks genuine enough to me; my only concern with it is the whole "debased gold" thing; your coin looks more coppery or silvery than gold, but I really don't have enough experience in the series either, to know how "un-gold-like" these coins can look.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis