Which part of London are you in?
If its either Southwark, Westminster or Kensington & Chelsea it will be up on that 3 story roof for about 48 hrs before you have the councils planning department all over you.
Unless the dish can be kept completely invisible from all round the building it will be removed. Also the likelyhood of you getting planning permission for anything bigger than a 1.2m dish is next to nothing.
Nano