Dash&Thump
Take a look at virtually anytime at LHR and you'll see large aircraft waiting at the hold for 10-15 minutes, burning incredible amounts of JET-A1. Look at BNN, LAM or the other holds and every day there's the majority of arrivals going round in circles for the usual 10-15 minutes or more. Again how much fuel burned?
The reason they have so many waiting and holding is to maximise the use of the runways.
BUT - if they build the 3rd, they will not just reduce the waiting times, they will add NEW rotations (to justify and pay for it) and so the waiting/holding times will stay the same.
I agree that reducing them now and saving zillions of tons of fuel should be a priority. But it isn't and won't be. My bet is that LGW will get it's second and the LHR 3rd won't happen - certainly not within the next fifteen years.