It is a London-wide phenomenon. London went into population decline after WW2, resulting in the "de-gentrifying" of many central areas as wealthy people left for commuter towns. The suburbs stayed pretty much where they were. The decline was only halted in the 80s, since which many of those people have moved back into the city, reversing the previous trend.
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Unless London experiences another massive demographic shift like that after WW2, the pattern will be one of continued gentrification in all areas, with no new slums.