BAA plc will immediately over sell it by oferring more slots than they can handle at the terminals, taxiways and stands (as they have done with the two primaries). So the capacity would have been expanded but the capacity issue would remian the same.
This is exactly where the problem lies - let LHR have a third runway and it will fill up. Yet Boris island might only have a similar capacity to an expanded Heathrow because it faces its own internal constraints too.
However, we shouldn't confuse
real demand with the
natural ability of hubs to bloat themselves because as they expand they bring in more routes and thus more connections so they expand further. As this expansion is fed by the less valuable transfer traffic, it massively over-inflates the true value such a hub would have to London Plc.
In terms of the direct demand for point to point routes into London as a destination and out of London as an origin, we still have a little bit more room.