I think the problem is that those resevoirs hold quite a bit of the Greater London and beyonds water supply. If you would have to build a new runway, you would have to loose them, and build new ones elsewhere, and i can tell you Peter, water is already scarce in the South East 😉
:EDIT: And not only would you upset those who have to move for the new runway, but you would upset those who’d have to move for the new resevoirs.
Both have and will create plenty of controversy