Yes, this is an exception to the rule in British English, although possibly old-fashioned and becoming obsolete. Some British people put "an" before any word starting with "h" and longer than three syllables.

EDIT I'm wrong here - I don't think anyone ever says "an Halloween celebration" for example. But I was sure there were other "h" counterexamples than historic.

for ESL people, its both.

For english people, as in actual english not from the UK, its a historic, unless its for emphasis.

There is/are some religious overtones, that are infused into this. Catholics used to (or still do) say "hay-tch" for h, where as non-Catholic would say "ay-tch". However that distinction is not so acute in england(unless you're in/near liverpool), but it might be in scotland and its certainly there in NI

Comedians often make jokes.

This is a common overcorrection. It's 100% "a historic" if you're pronouncing the "h" (which everyone does).

People who say "an historic" are wrong - they're imagining there's some exception that there isn't.

You can easily tell they're wrong because who says "an history lesson"?