British English and American English don't differ that much that one cannot understand the other. The biggest difference is the pronunciation of words (accent) and certain terms, but apart from that the two are mutually intelligible.
I would keep in mind that some regional accents can pose a problem, (Southern American) or(Northern English), but these are considered non standard forms of the same language.
An educated Englishman can usually understand an educated American.
Parts of Scotland and Ireland have very thick accents that may take awhile to decipher, but are usually understandable.
Australian, New Zealander, S African are easy to understand.
Standard Caribbean English can be easy to understand, but non standard can pose a problem.