if you want to maintain separate versions for British and American, I recommend you find a native British English speaker and a native American English speaker to proofread and edit your content, If your audience is primarily British, or if you have no access to a native speaker of American English, just do the British version; if the contrary, just do an American version. You will not learn enough of the rules from any one "source"; just do your best, then have it edited. There are too many subtle differences between American and British English. If you can set your spell/grammar checker to use a British or American English dictionary, that might be a good start, at least for spelling. But it won't fix all your usage and syntax, let alone ensure proper use of idiomatic expressions.