The national statistics pages look a bit daunting. A few days ago I think I posted a link to this FoI request from HBS school in North London, which in years 7-11 seems to be about maybe 75-80% Indian, something like that - see below. This is not a number that would particularly surprise me, though I only really have direct experience of the South London places. Precise figures vary by school, 75-80% is probably atypically high. Many would have a much lower figure than this.
I don't think it's the hugest deal ever, either way provided your own culture isn't one that might specifically for whatever reason clash with the dominant one.
As PP says, there is probably a social element, e.g. I'm not sure you'd expect a hypothetical school-organised 'battle of the bands' held at one of these places to be a Woodstock-esque affair. But to a very large extent kids are just, y'know, kids.
If it matters to you then for your peace of mind it might It's worth checking what sports teams the school runs and so on - in my experience it's the norm for these schools to make quite a decent effort, maybe slightly more effort than most comps in similar areas do.
If this matter to you it's worth checking their detailed A level results, you'd probably find that, say, art, maybe also one or two other subjects, are relatively less popular than at the national level at the expense of STEM, though provided they do run the subject then e.g. being in a small class might be a good thing [or not].
https://www.whatdotheyknow.com/request/pupil_diversity_3#incoming-1956757