To validate some of what Dallas Dude said about slots and overcapacity at airports one can look at EWR as an example. Even though JFK was never slot controled a lot of Intl. carriers either switched their flying in to EWR altogether (SAS) or moved some of their flying in to that airport. This happened in the late Eightees and early nineties because of the long delays out of JFK on the afternoon push to Europe. The same will probably happen at Heathrow, and already has to a certain extent. Now EWR has the same delays, but that's another story.