What an interesting question. Off the top of my head, I would say it's probably Paul who's retained most of his "Englishness." I think I read somewhere in his book that he was rather taken aback at how "American" John's manner of speaking seemed after moving to New York, as though Paul were somewhat uncomfortable with that.
I can see John and Ringo embracing outside customs and ways more easily, whereas I feel that Paul would cling on to his English nationality more. I'm sure he's gotten a healthy dose of American culture, however, from his love of American music and from Linda and Nancy. As for George, I think his heart and being truly belonged in India, although I don't suppose he ever lost his dry British sense of humor.
Just my two cents!