Actually it's 1000000 m^2 per km^2 not 1000. So My equals roughly 2300 m^2 per person.
So if you had an average of 5 floors per building underground or over ground and used 6% of the land instead of 9%. You could redevelop London to have more spacing capacitate with more organic design underground over ground structuring and have mini cities and leave the rest of the space for business services houses heritage ect.