...
A historic is more common in both American and British English, but both usages are sufficiently common to be considered correct.
A well known grammar rule says that we should use an before vowel sounds; for example, an accident, an item, an hour. We use a otherwise: a book, a hotel, a university.
Notice that we say an hour, not a hour. The choice of a or an is based upon the sound of the word, not the spelling. Hour sounds as if it starts with a vowel sound (ow); hence, we use an.
Following this rule, we would say a historic, not an historic because (for most speakers) historic doesn't start with a vowel sound.
Words of three or more syllables that start with h are treated differently by some speakers, though. (This may be because of the tendency of some regional accents to drop initial Hs.)