They will be lucky to put a new runway in for £13bn, sure the runway on its own won't cost that but it has to be linked to the main airport, so that will be major work to get Bath road under it at least twice and then there will be terminal 6 or 7. After that there is compensation for relocation and additional sound proofing for however many?
Another air traffic control tower, ILS power supplies, water sewage, road links, rail links, think £20bn?!! a new site won't cost anymore and you have the growth to cope with it.
It will only make any sense if LHR is closed other wise most operators will stay put, BTW it will probably finish LCY off as well, but the land will be worth a fortune by then
$million dollar question, will happen, i doubt it, but LHR will get R3 is my bet