Think of this the other way around too. A Frenchman wants to go from Lyon to Exeter. At present this is not possible through LHR as there are no Exeter-LHR flights. (In fact, to get from EXT to LHR, you have to go via CDG !). So the only option is Lyon-EXT via Air France and CDG. So UK airlines and UK Plc loses out yet again.
Silver, not the best example, but let's run with it.
Why are there no flights from EXT to LHR or any other LON airports? Simply not worthwhile, far too short a sector. So going via CDG would be pointless, we have these things called trains. Also, for that route in question, I'd do LYS - CDG by train, then CDG-EXT.
Now BE have a major presence at EXT, SOU, BHX and a few routes from NWI too. All of these airports are too close to London to feed it by air - but apart from NWI, they are also to the west of London, so any fast rail link is going to work far better going into LHR, not Boris Island. Same also for BRS, BOH etc.
So there's no point in bleating about the hub traffic 'lost' to CDG or AMS - London would never handle them anyway. And I also totally refute your point about interlining - people want to go direct, the Q400s interiors may look a bit cheap and nasty, and some people will grumble about turboprops, but most people just look at the headline price and go for the direct route if it is there. You are right in that sometimes an interline fare is cheaper than a direct service on a loco, especially during busier terms, but these are the exceptions not the norm.
Most pax don't give a damn about BHX's crosswinds either.
Now to go back to what I asked you about the island size - I will continue to base my critique on the proposal as it stands, not the alternatives you have suggested, but are not in a position to cost.
So I put it to you that, in their present form, Foster's plans might provide 2 extra runways, but they do little to provide enough terminal space to handle the extra flights. Therefore, it is hard to imagine the new airport handling over 100m pax pa (lets say 105m for good measure), and even if it does, they will have to use the largest aircraft available (ok, A380 is longer and fatter than a Q400, but they both have wings. As A380 is double deck, it surely offers the best pax# to space ratio?) - a method of adding capacity which LHR could also use.
Now do the maths - even if you were building on terra firma - why would you invest £xx billion in infrastructure to grow capacity from 70m pax pa to 105 m pax pa, when you are spending as if you are growing from zero? In other words, for every £3 spent, only £1 is going on new passengers?