When we went for the St Helen's assessment last year (we didn't get in BTW) we found that the vast majority was Asian British, not Indian.
Believe me, there is a huge difference.
We moved here recently from India. My English is accented and I definitely appear (and am) "foreign"
I found that there is honestly very little difference between the Asian British and the White British - they speak/think/act/dress the same. Most of the "Indian" appearing people are actually 3rd/4th generation British, migrated from Uganda/Kenya and the like. So the Indian connection is very minimal. Yes, during the festival time (Oct/Nov) there might be some talk of Diwali etc but to be honest, the rest of the year there's nothing special happening.
It must be incredibly hard for the Asian British to "convince" people they are actually British, regardless of the colour of their skin.
What I found was cliques of British parents (White and Brown!) and non-British. I felt very out of place. But, I expect this as a foreigner and do not hold it against anyone. Of course it's normal to hang out with people you have more in common with!
But it would have affected my child's social life for sure. People are friendly but they do judge/look down on you without knowing anything about you. I remember feeling very hurt and upset the first time I encountered some patronising behaviour from an Asian British woman - I didn't expect it! Silly isn't it - all bad treatment doesn't stem from "colour-racism", it's also when someone perceives you as being different hence inferior.
Pushiness is not very attractive especially the culture of secretive coaching /tutoring for assessments or to stay ahead - sounds awful.
But it's not only Indians who do it! And not all Indians do it. We did not tutor our DD for the assessments, she doesn't do Kumon or any academic classes.
Most of the Indians I know have a strong focus on academics. That's the one way to survive and succeed in India. I consider this a very positive trait in the classroom - your child would automatically try to do well, especially if they are intelligent and well-motivated to be in StH in the first place.
Yes, there are many many languages in India proper. But the cultural mix in this part of town is such that most of the Indian families know Gujrati/Hindi and the children know it too. So it might happen that the kids talk to each other in a local language. But unlikely for it to persist beyond Reception. Most Indians I know have a home language/outside language=English.
We speak only English at home, my child doesn't know any "Indian language" even though my husband and I speak about 5! I would never speak in an Indian language outside when someone else present might not understand it - I consider that quite rude TBH.
I'd just like to add that it is lovely to see the little ones grow up in an environment of multi-culturalism and colour blindness. My DD's best friend is from Nigeria - I had a lot of preconceptions about Africans - re work ethic etc, I am extremely ashamed to admit - all due to ignorance! Now I know how wrong I was - and how wonderful all people can be.
Yikes, this turned out long! Sorry