There are plenty of black and Asian people among the middle classes, sp. They don't all live in inner city council estates in the same way that not all whites live in mansions or leafy suburbs. The fact is that the Proms are not part of the culture of most ethnic minorities. It has nothing to do with class. They have no interest in such events because they are not part of their culture and they are not inclined to make them so.
Football is a different matter. It is played and watched throughout the world. Travel to India and you will find that, so long as there is no cricket to watch, youngsters will be watching Chelsea vs Manchester United on their satellite TV. Those who travel to the UK will obviously continue that interest.
There is no reason why those from ethnic minorities should take an interest in the Proms. Similarly there is no reason for English people to take any interest in Diwali. In the UK we do not have a multicultural society. We have a collection of different cultures existing in the same place. Indeed I would argue that the term is contradictory. From my dictionary, the definition of “culture”:
“the customary beliefs, social forms, and material traits of a racial, religious, or social group; also : the characteristic features of everyday existence (as diversions or a way of life} shared by people in a place or time”
Note in the first definition the singular “racial, religious or social group”. Not a multitude of different groups brought together. Note in the second definition “...shared by people in a place or time”. I cannot imagine that anybody might support the argument that the Bangladeshis in, say, Tower Hamlets share “the characteristic features of [their] everyday existence” with any of the few people of non-Bangladeshi origin who live in that area. They live completely different lives by completely different cultures, not shared at all.
As an admittedly very racist comedian once quipped, “Just because a dog is born in a stable, it does not make him a horse”. He said it to get a cheap laugh, but I think he had a point.