There is also the little matter of the written Standard. The way we say something and the way we write something are different, but written English is much more level than spoken English. I can understand things people in the UK write (be it on-line or in books), and I can understand Englishmen when they speak. Slang, etc, are going to be present in any spoken language, so that's true of both communicating parties.

Cheers,
Nick