I have heard about a group of people in London I think who go around planting up wasteland spaces.They do it a lot at night. :)
Allotments,especially urban ones can be found in all sorts of odd places.
Good on you for trying with your idea but you would probally be happier trying to get an allotment,otherwise it could turn into a battle and to me my allotment is all about enjoying myself and learning to be as skilled as I can.