Don't be so sure. Heathrow with an extended runway to twice its length is looking a much more attractive option. (as is the case in Madrid I believe)
The Heathrow split-runway proposal is included in the options purely as a makeweight. Nobody takes it seriously, which leaves LHR3 and LGW2 as the only proposals that are likely to survive to the next stage.
Technical and safety considerations aside, extending 09L/27R is a non-starter because to increase capacity, mixed mode would necessary
on all rwys. This would be very hard/impossible to sell to the local residents.
The only one of the three proposals from Davies that will actually resolve the problem is the "northwest" rwy option.
A second rwy at LGW increases capacity there, but not where it's needed (i.e. at LHR) and does nothing to increase the UK's
hub capacity.
As for a "makeweight", taking another look at the estaury option should also be in this category.