Also with CrossRail now being dug (at long, long last!) how about taking LHR out of that? One of the (many) justifications was that CrossRail would get more pax into the airport underground, rather than by surface.
Of course, they could just not build the spur but - really? Successive UK govts have no idea on the topic of railways and airports (probably ports as well but I don't know anything about them!)
As is often said, all the folks who will have a problem with an expanded LHR get to protest but the folks who will benefit have no support group. because the promoters are those who will benefit financially, they are derided as being only in it for the money. It's Catch 22 every time in the UK.