Airports Commission and the "regions"
The biggest impediment to long haul expansion, trans-Atlantic excepted, at BHX, MAN, GLA, etc. is not Heathrow expansion, but EK.
That carrier saw the opportunity, took it, and is now very well embedded at the largest non-London airports with a dominant position in long haul.
Despite most journeys being the long way round and requiring a change of aircraft at DXB, EK has gone from strength to strength. The same applies to EY and QR at MAN.
Carriers wanting to offer direct flights to Beijing, Shanghai, Tokyo, Bangkok, and Johannesburg, as mentioned above, and other destinations, have to take on a long-established and sucessful operation.
The question is: do they have the stomach for it? Bearing in mind that most pax are happy to take the long way round via DXB (else they would go via AMS, CDG, FRA, or LHR (except BHX in the case of the latter)), is there sufficient premium business to make direct flights viable?
Heathrow with two, three or four rwys is irrelevant in this.