A tube would of course be great (especially given the terrible Sunday and evening services on the train), but it will never happen. I'm sure tunelling through South London is technically possible these days, but it would be such a mammouth project, it's almost unthinkable. There's no such thing as an area which is nice, affordable, close to the centre, and with a tube. Because if there was, everybody would want to live there, so it wouldn't be affordable for very long. If you want somewhere affordable with a tube, East London is the way to go (but most of it is not very nice).