Actually the English spoken here in the U.S is closer to what was standard before the 19th century. England became isolated after the loss of its empire, so a new dialect formed there.
That's not true. Elements of the way the language is written in the US (for example the use of -ize rather than -ise) are truer to earlier forms of the English language, and I'm already on record as having no issue with that. The version of English that is spoken in the US is wildly different to its predecessor though. Languages grow and develop, and American English is an entirely legitimate branch.
That doesn't change the fact that "an historical" is a correct usage of the language (as is "a historical", if you prefer).