Silver,
It really is not the same thing to compare Dutch reclamation methods, which involve reclaiming land behind dykes, but otherwise surrounded by more land, with the construction of a completely new island in water some meters deep.
Now I don't have a Thames Estuary contour map to hand, but I put it to you that your island would be several times deeper than Foster's only partially offshore proposal at its deepest point. I still question the merits of combining Thames Barrier and crossing in same structure, as optimal points for either are not necessarily in the same place, but Foster's idea is at least much closer to the mark than yours, which could be at least twice as long as his, plus the extra cost of building at such depth, and having more depth changes in the sea bed - not so easy if the crossing goes below the barrage (as Foster's would). There is also the question of how the various Thames ports would operate if Silver barrage was closed? Locks in such a structure would be prohibitively expensive.
Now as to the runway separation - indeed I don't have a crystal ball, but I do have a reasonable knowledge of European politics, as do most contributors on this thread. So UK domestic and Schengen would still be separate zones. And the runway still doesn't care about the nationality of the pax using it, just a question of whether takeoff / landing can be achieved, yes or no. Yes, I agree, you could build shorter runways to serve the local traffic, but better to use the island terminal + 2 runways each side widely used elsewhere, and proposed by Foster. And even if you are going to use 4L + 2S, surely your island needs to spin the other way round - most of the local traffic would cross the LH otherwise.