British English is the form of English used in the United Kingdom, spoken British English varies considerably, reflecting along history of dialect development amid isolated populations.
American English is the form of English used in the United States. It has been developed widely and lots of pronunciations and spellings are different with original British English. E.g. “fertiliser” isBritish version and “fertilizer” is American version.
But their written forms are not so noticeable in difference.