I'm sure part is about slots but I am also sure WW realizes that people don't want to fly via LHR like they did 10 years ago. Something a new runway won't change and he will have to deal with. Not sure people from Ireland will fly via LHR to fly east either. BA drop their 8 daily when required but suspect a few other frequencies will be up the peaking order.
I believe RY3 will be given the go ahead, if the Tories are about the "economy" they will clear it. I'm sure LGW will manage to fill their new runway with bucket and spade operators but we know major players in the market won't fly from there runway or not.