If you look at T5 the vast majority of the food places are after security.
That's all very fine if you fly from LHR, but although there are restaurants the other side of security they do not offer a takeaway facility. (I don't include fast food)
Gordon Ramsay's restaurant do all sorts of options packed in a box.
Very nice too! What about the health risk of carrying food around at room temperature? Additionally are you really going to ask the cabin crew to heat up your vegetable curry or Bhindi bhaji?