in answer to why London as opposed to the south! Actually no reason why it has to be London, but London in all its guises through the years has been the team that has succeeded. Rugby league has been played all over the south and still is, how far Gloucester/Oxford/Hemel etc can go is yet to be seen. London is pretty much the focal centre of the "south" as you put it so logic would suggest London as the gathering point and the place to have the professional team. The potential!!!!!!!! audience in London is massive, it was just never exploited. Don't get me wrong I would love Glos/Oxford/Hemel (Bristol/S wales etc etc) to all be able to compete at the top level but as is constantly talked about the playing talent pool is not big enough to sustain that many clubs in the top flight.
So in essence expansion has been tried outside the "heartland" and up until now the only ones that have managed it are London and Catalan. It has been tried elsewhere Paris St G/ Kent Invicta/ Nottingham/ Cardiff/ Sheffield are even without a home at the minute and that's on the fringes of the traditional home.
So no reason why it HAS to be London but one top flight club in the south London based club is the logical place.