It is difficult to spell out what cultural difference is, yet cultural difference is not the same as ethnic difference. African-Americans, for example, are not culturally homogeneous; yet in some politically important respects they are different from both Anglo Whites and African-Hispanics.
But upper middle class English-speakers have much more in common as a class than a difference in ethnicity, Black, White, WASP, Polish, Irish, etc. might suggest. Ethnicity is a socially acceptable form of cultural difference because it involves little if any real cultural difference.
Ethnic leaders are leaders precisely because they have been assimilated enough to want to trade off cultural difference for a piece of the social pie. To the extent that other members of their groups are willing to make similar compromises and get involved politically, to that extent will their group achieve ethnic status.
The alternative is to risk being treated as deviant beyond the range of acceptable variation and, like the Amish, or any number of small religious sects, invite the importunities of the State.
To examine these issues further, see Edward G. Rozycki, " Pluralism and Rationality: the Limits of Tolerance. Why public schools fail.