We need a new runway at both LHR & LGW, by the time LHR R3 is open LGW will be full!!
They address different markets with some overlap, the physical ground and layout constraints will always mean that tarmac time will be higher at LHR than LGW, less of a problem on a 9-12 hour flight than a 2-3 hour flight.
I don't think that the Government will sit on this for two long, they know they need to get all the difficult stuff cleared within a 2 year span, most people forget about what happened 3 years ago or who was to blame, Boris & Zac are problems for sure.
Politically LGW would be easier for the Government and require less public money but would not address the hub issue. The wealth in the South is biased to the South of the Thames so forget STN
There is no easy answer to the M25 and certainly no cheap one, the expansion to 5/6 lanes means there are now 5/6 lanes of stationery traffic at some time most days, it would be easier to build the runway 3m higher than sink the M25 by 3m.