They moved somewhere else. Simple as that. As Brian said, "white flight".
I'm sure they haven't ALL done that. Not everyone can afford to. But if you're going to invite immigrants from the non-white Commonwealth to help rebuild after WWII (and that's what was done - Windrush and all that), inevitably you're going to get areas that don't look full of white people any more. "Birds of a feather flock together" - it has to be admitted that people feel happier with other people most like themselves, same culture, somewhere they can get their kind of food... I know it is unpopular with "politically correct" people to say this but I say to them, look around you, you can SEE it.
I'm deliberately referring to skin colour because by now most of the non-white immigrants ARE English with British citizenship, and they've had children and grandchildren who were born British. White does not equal English, not any more.
You could ask the same about my own native London suburb of Southall, which now looks like "Little India" and has been called that! It was a running joke in the sixth form that I was the ethnic minority as I was the only white person doing science A levels. My parents always had a plan to retire further out to be closer to friends from work, Dad unfortunately died at 58 before they could do it, but Mum did it and of course I had to go too as I couldn't afford my own place to live (and nor could my sister). And that's where I am now.
(Not with Mum - I did eventually save up enough to buy my own flat, and now she's on her own she's moved into a retirement village a little further out, not even in Greater London any more, where she's having a whale of a time being with lots of other people her own age. The big downside, as she says, is lots of funerals as everybody's old so she knows the local crematorium rather well!)
That's it, really - people move house.