[chrispenycate]

Well., I don't speak American English, although I understood it across most of the United States (some problems in Texas and Louisiana) And when I spoke British English to them, they generally understood me (as far as anyone ever understands me. I made an effort not to use any long words)
However, one of the crew came from Glasgow, and thus, presumably, also spoke British English, but had noticable problems in making himself understood. Obviously it is not merely the Atlantic which causes problems.
Compared with the divergence of Quebecois French from the root language, American and English have hardly separated at all, and I put this down to Hollywood and television maintaining the information flow over the last century, but I could be completely wrong (difficult to run a double blind experiment)
But regional variations within the British Isles (even if they're officially not written variations) are at least as major as the transatlantic split.