The iPhone is great, but its autocomplete isn’t the best. It gets frustrating when you are an English speaker / writer. (as compared to an American English speaker / writer).
You can enable the British English dictionary on iPhone. Once enabled your phone will use the British English for autocomplete and ‘define’ :)
To get the British English dictionary:
You can then remove the American English dictionary. Now, none of the words you type will get replaced by an incorrect spelling. (eg centre will be correct)