Religion vs culture is a subtle but important difference. Religiously, India is not very diverse, but culturally- that is, along ethic, language and racial lines - it is probably the most diverse country in the world. I never claimed the Brits united India culturally (except perhaps over a shared hatred of the colonizer?), but rather administratively (which continues today)

There is literal process studied by sociologists called Brahmanization where local elites were coopted and admitted to caste