[pdurrant]

Originally Posted by thinkpad

I'm confused, English is not my native language.

Seems to me that "a" is the preferred preference:
http://www.betterwritingskills.com/tip-w005.html
http://grammarist.com/usage/an-historic/

Your first link has a graph showing a preference for "an historic" up until the 1970s in British English writing.

And in spoken (British) English, it's a lot easier to say as "an historic" instead of "a historic".