Summary
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.)
Google's N-gram viewer allows us to see how this usage has changed over time. Here's the use of "a historic" and "an historic" for US English over the last century:
(Click for more details.)
It's clear that American English has shown a strong preference for "a historic" since the forties.
Here's the chart for British English:
(Click for more details.)
British English also shows a preference for "a historic", although not as strong. This preference is also more recent, perhaps reflecting the influence of American English brought about by the Internet.
