This sums up my own thoughts too. I grew up in a multicultural suburb of London in the ‘80s and ‘90s and loved it - it felt like a genuine, relaxed mix of cultures all intermingling, but still with a sense of British culture being the dominant overarching ‘norm’. In hindsight I recognise that since I attended a grammar school and lived in what was a fairly leafy area the shared social class, aspirations and educational backgrounds of the peers I mixed with probably hugely aided social cohesion at the time.
I first became aware of the parallel, atomised communities as a teacher in some much more deprived, highly multicultural suburban London schools from about 2007 onwards, but it wasn’t directly affecting me so I just viewed it as an interesting and slightly depressing phenomenon - it manifested as groups of predominantly Muslim heritage choosing not to engage with the overarching culture, opting out of all shared community activities, not learning to speak English (or in many cases allowing women access to English lessons at all - multiple translators were provided for parents’ evenings etc) and bringing foreign political disputes into the classroom (as a rookie teacher I was having to contend with statements like ‘We hate the Jews because they murder our brothers’ in Year 2 RE lessons on the basic tenants of Judaism).
Then from about 2015 onwards my own, much leafier area of the same London borough started changing dramatically, with the demographic changes accelerating during the pandemic lockdowns (I assume this must have been the ‘Boris Wave’). Rather than being a true melting pot, a couple of cultures now had sufficient numbers to outcompete the rest, and these have now established themselves as the dominant cultures in the area, with most community events, new shop openings etc being aimed at this demographic. There is very little sense remaining of any overarching British culture, especially as the council leaders all belong to the same demographic. At this point our family decided to relocate, and we have recently moved out of London to a more traditional, homogeneous area which feels much more familiar to us, and where we feel much more at home.