The cross runways are as has been pointed out irrelevant because they are now under terminals and even before that intersected with each other making use of more than any two at a time difficult to say the elast although I am sure it has been done on occasion it would have probably only lead tyoa tiny increase in movements.
However the land north of LHR has never really been developed and there are surprising ly large areas of open ground between the A4 and the M4. development along the A4 is much the same as 50 years ago aside froma few hotels and isa sort of ribbon development parrlaleling the road. Problem is that there are three villages, Harlington, Harmondsworth and Sipson which are agin essentially ribbon developments and old lanes that cross the area north to south to north from what was the old Bath road and at least one, Sipson, would pretty much have to go to build a third runway but that is the History of Heathrow. named for the village sacrificed to the airport along with Hatton , parts of Stanwell and Stanwell Moor , Yeovany and Poyle . Sadly in a crowded country somethings have to go to make way for progress and my personal view ios that the victims are more afraid of being screwed over by biased compulsory purchase prices than the actual loss of habitat, although there are a few nice old buildings these places are not that desirable being in the middle of a flat desolate plain with poor public transport links and lots of the disadvantages of being very close to a huge airport.
But in Uk we just do not have the stomach to do anything at all like this, if the amount of Government and Civil service effort that was put into covering up the Hillsborough tragedy was directed at Heathrow expansion the runway would be ten years old by now.
The other ridiculous element in this is the idea that it could be Gatwick or Heathrow and of course it cannot. Airports are like railway junctions , lots of people need to interchange so building a second runway at Gatwick cannot effectively remove the case for expanding Heathrow, it will just see more and more and more people using AMS CDG and FRA instead.
But perhaps thats the Boris master pan, leave the EU to cut short haul travel and make it difficult for anyone to get to Britain at all to deal with immigration