As previously mentioned a new airport in the thames estuary is such a difficult and expensive proposition it practically makes it a non-starter. Think cost, logistics and then airspace changes that'll be so heavily contested it'll make the current legal challenges to proposed runways seem like minor squabbles.
Also who's going to pay for it with BAA investing so much money in LHR they won't be interested, the government is hardly awash with cash and anyone with a spare billion or two looking to get involved in aviation will buy Gatwick before they pour it down a black hole in the thames estuary.
Northolt similarly has its problems, it is extremely close to LHR and the fact that its runway is not parallel to heathrows' means that integration of more flights becomes more tricky. Other obstacles include the fact that the RAF are incredibly unlikely to sell it as it is an extremely useful base for them and they can simultaenously make a few quid out of it. If I remember rightly landing fees and movements are strictly regulated by the CAA as they were lobbied hard by farnborough and biggin as the RAF were undercutting them and taking business.
Whichever way you look at it another runway at one of the existing airports is the more likely option, although how likely that is is another matter altogether. The country as a whole needs to have a serious debate over what role air transport will have in the future in this country and whether we want to remain a major international hub or not.