NHT would be a small scale operation with only domestic and "near abroad" traffic.
I don't see why this should be the case, with a circa 1700m strip, all non uk based (for maintenance purposes) shorthaul could use NHT more or less without restriction. As has been pointed out, a high speed underground link for the 5 miles to LHR, would effectively make this part of LHR, not separate from it. The high speed link could be an airside one, making NHT just another pier at, say, T1/2 at LHR.
As Fdf says, just what are we waiting for?