this is what bugs me. One of the things that used to be amazing about London (and still is, but to a lesser extend) is that there weren't any rich ghettos- Everywhere was mixed to some extent. It is, as you say becoming less like that, which is really, really sad. London is losing one of its defining characteristics.
that is only a problem because of the above- and then people become the enemy, as opposed to just other people who happen to live there. Sure lots of the (not so young) professionals buying up Brixton are perfectly decent people, its the policies that are to blame not the people