but there is ample land to use when it comes to building HS2
HS2 is a strip of land about ten metres wide by a couple of hundred kilometres long, so a couple of square kilometres. London has a population density of about 5000/sq.km. So HS2 would provide room for 10 000 people.
new runway for an airport
A full-size runway at a big airport is 4km by 50m, so 0.2 sq km. That's room for another 1000 people at London density.
So you've killed off two major infrastructure projects, the larger of which is about the only way we can provide sufficient commuter capacity into London from the north (the main reason for HS2 is to make capacity on the WCML) and found room to house about 11 000 people. Current housing starts are about 140 000 per year, so housing so, what, about half a million people? So the land needed for HS2 and a major runway build would provide an additional week's worth of building land, once.