For words in which the spelling differs between British English and American English, the BBC Learning English program includes the spelling for both. In most cases, both versions of the word are acceptable in either language.
Words that end with "ise" or "yse" in British English typically end with "ize" or "yze" in American English. British past tense words also often end with the letter "t" when American English words end with an "ed." In all these situations, British English also accepts the American version of the words.
Exceptions include British words that use "ou" or "re," such as "colour" or "theatre." British English doesn't accept the American equivalents "color" or "theater."