The rule in England for a/an and h has for many years been the same as now appears to have been adopted in the USA - that the pronunciation determines whether it is a or an - but of course pronunciation varies.

Fowler (original) yet again: A is used before all consonants except silent h (a history, an hour); an was formerly usual before an unaccented syllable beginning with h (an historical work), but now that the h in such words is pronounced the distinction has become pedantic, & a historical should be said and written.