It hasn't been 'debunked'. It is something that happens time and time again. London has a white majority yes, but there are several parts of london that quite clearly do not, and there is massive segregation. For example I spent parts of my life growing up in Plumstead, back then it was what I would consider 'multicultural', as in that there was a clear immigrant population but not extreme. Nowadays however the only family member I know that still leaves there gets attacked because she is white. The British people have left and the few that are left now get attacked, multicultural segregation at its finest.
I bet my life that Blackburn still has a white 'majority', but you can watch that Panorama program and see that it has obvious segregation, which according to one 'eye-witness' is worse than South Africa during Apartheid.