Basically, rooftop farming is baked. Harrods now even sells rooftop veggie patches [link] to those wealthy few who can afford them, millionaires who would like a restful allotment on top of their skyscraper, but can't countenance being surrounded by commoners. There's an experimental rooftop farming
collective in St Petersburg [link] who recycle their apartment building's compost, and use it on the top of the building. A US company sells 'robotic greenhouses'[link], fully automated systems for skyscraper roofs. What I'm proposing, then, is not technology; I guess it's a business model of sorts.
In London (and other cosmopolises worldwide, one assumes) organic food still gets premium price, but is old hat. The new thing is 'local' and 'carbon-neutral' food. In central London, however - where is 'local' that grows food? Nowhere.
Except that there are a heck of a lot of flat rooftops and (from experience) abandoned back gardens in London. The idea would be to approach the owners of said roof-tops and gardens, and for a modest monthly fee purchase a license from them to use their land. Garden-licencees could be provided with an allowance of free vegetables; rooftop-licencees would have the beneift of added insulation and the, er, aesthetic benefits of being 'green'.
The vegetables grown would initially be simple ones needing minimal irrigation or maintenance, and ideally seasonal (so as to eliminate a need for greenhousing). However, for long-term leases on inner-city rooftops, more complicated greenhousing systems could be used, including (perhaps) PV cells or wind-gens for power, providing electricity for ventilation, irrigation, even nightlights. Insofar as was possible and economically efficient, such 'technified' rooftops could be computer controlled, as per the 'robotic' greenhouse link.
Provided licenses assured access to the rooftop/garden at agreed times, maintenance could be undertaken by gardeners (a simple training manual could be easily developed) driving electrically-powered 'floats' around the city, arriving every morning like milkmen to turn the soil/weed/etc. They could also deliver increasingly-popular veggie boxes[link], as they do so.
Obviously, no matter how low your roof/garden rent, there's no way you could compete with commercial farming in rural areas. So the business would be a niche market; exploiting the prices people in places like London will pay for luxury organic food, grown locally with minimum - near zero - carbon-cost from seed to plate. Not even veggie boxes come close. I can just see clients hip cafes in Hoxton and posh eateries in Kensington grinning smugly as they tuck into a rocket salad grown within a mile of them.
[Disclaimer: This is something I've been mulling over for a while, and then I saw [scottinmn]'s idea [link], 'Rooftop farms' - rooftop farming, en masse. I started to annotate with my ideas, but it kinda spiralled out of control, got overly long and far away from the original; so I'm posting it as a 'new' idea, but I want to make clear that I wouldn't be posted if Scottmn's idea hadn't prompted me, so kudos to him, and I'll re-merge this back as an annotation if this is bad HB etiquette.]