What about having the "American" English becaming the "Main" lanuage eg. no more "u" in words that dont have them like Color and Honor. And have the American nouns over the English, Fries instead of chips, elevator instead of lift ect.
This can be done before 1751, by having the colonies more populated then the British Isle and having colonist, traveling to Britain more easily.
British English being "main" has less to do with American colonial population, and more to do with half the world having been ruled by the British, who imposed their education on them.
Even then, a lot of American variants are becoming more popular all over the world (and conversely, some Britishicisms are gaining popularity in the US).
Another issue, of course, is that there was not and never will be just one English dialect. Someone from Appalachia speaks a very different dialect from someone in Los Angeles, who speaks differently than someone from Glasgow, who sounds different from someone in Delhi, who sounds different from someone in Sydney, and so on and so forth.