The distinction between the indefinite articles a and an should be straightforward: use a when preceding a vowel sound; use an for everything else. Thus a one-time event, an epoch, a university, and an umbra.
British and American English sometimes differ on voicing leading letters. While American English prefers an herb, British English writes a herb. This phonetic difference does not continue to all h words. Historic, although mistakenly written following an, should follow a in good writing, which does not care for poor local dialects. H.W. Fowler and Bryan A. Garner both advise against the silent h interpretation. Prefer a historic, a hypothesis, a hereditary, etc. in both British and American standard written English.