You keep using capitals to describe "White Incomers". You really think that someone's skin colour defines how gentifying they are? What exactly is your point about race and being 'cool'?
The "black" people I know (born in Brixton, second generation Jamaicans) who have moved out of Brixton gave the following reason: "I don't want my kids growing up round here - there's too much trouble". They afforded the move when them and their partner both exercised right-to-buy and/or sublet their social housing. Not exactly "middle class" as he is a builder, but at the same time what defines him isn't his skin colour - he personally has aspirations just like a lot of people his age (ie late 30s, young kids) to live somewhere "nicer" and really doesn't give a shit about 'cool' nightlife or having a tube line to central London.
I am not saying that this represents all "black" people or all long term Brixton residents who end up moving away - in fact everyone has their own reasons, and I really dislike the way you want to racialise everything without even trying to offer any more analysis of culture, communities or anything else other than "Black" and "White".
I do remember reading this tho':
More foreigners than ever are coming to London, and more Britons are leaving
and
Black Britons flee the inner cities
These two articles alone show that things are far more complex than you'd like to paint them.
I know you don't actually "do" evidence, but maybe you'd like to explain what exactly you are burbling on about?