Rooftop gardening of fruit or vegetables or Purloining other peoples' roof space

It was only the other day that I was gazing down from my luxury Kennington palace, wondering whether I could purloin roof space from my and neighbours' rooves, in order to grow some flowers or vegetables. Everything looks very grey when gazing over rooftops, and some of the roof spaces get a lot of sun...

But it seems that somebody else has been thinking the same way. A Telegraph article tells of roof space to be converted to vegetable gardens for feeding Olympic athletes. I suspect we might be too far away to participate, but I really like this idea.

I think there may be issues re. accessing rooves. Our roof is communal, and requires a ladder to reach it. Not sure who I'd need to get permission from (not that that would stop me). Also, to access my neighbours' rooves (some of these are light commercial enterprises) would need permission, as well as odd access, and I don't know whether some of the roof space could support the weight of soil and pots, but it is a thought that I don't want to entirely dismiss. In the meantime, I should really get in touch with the local Guerilla gardeners.

Does anybody know whether there are any types of vegetables that can be grown through the winter?