One of the great things about living London (as opposed to the sticks) is the dynamism of a big city - things change, develop, people come and go. Brixton is a good example: wealthy at the end of the C19th then becoming progressively poorer through to the mid-C20th century - which must be a reason that immigrants with little money came here, in the search for cheap housing. Things move on, the immigrants who came in the 1950s and 60s have retired, maybe made some money on their homes if they were able to buy, some decide to move on, and new people move in. It is the eternal cycle of the metropolis, and it would ruin London if any group (rich, poor, black, white) were able to stop the clock at any point in time and say that from now on the tides of change were halted.