If LGW is such a realistic option, then why have so many long haul carriers upped sticks and moved ops to LHR post Open-Skies? LGW at the mo is mainly LoCo and charter.
It's the status of and brand of LHR that ensures it's popularity with airlines. We know it as chaotic, at capacity and in need of investment, but it's popularity is due to the same reasons that Kennedy is more popular than Newark, de Gaulle vs Orly, Malpensa vs Linate etc. The secondary airports have a role, but expansion will be concentrated at the most established.
LHR will get it's next runway, irrespective of the party in charge, and I would expect LGW to get another one in the fullness of time. UK Plc simply doesn't have the funds, or the stomach to build a an estuary airport, and it doesn't need to.
Northolt however.......you don't know west London very well if you think that's going to happen!