Walnut
I believe this is a very sensible idea, It is much cheaper than Boris Island and much closer to either the capital or the current main London airport.
It should be purely domestic only, helping the immigration headaches, and have a high speed monorail transit to LHR terminals. ie a loop through the central area T4 & T5.
The runway is long enough for the operations envisaged.
People suggest that distances are too great, but just go to Atlanta for example. Boris Island pax, even with the speedy communications envisiged would take far longer to reach the capital & most importantly it is out on a limb re its catchment area. Imagine having to transit the capital to go to almost anywhere else in the UK.
The HS2 whilst difficult to justify would make more sense if a terminal was created at Northolt, so traffic could come up from LHR via the monorail to then connect onwards.
Lastly I believe this project could be up and running within 5yrs