I doubt that, given the choice, Delta would withdraw completely out of London Gatwick in favor of London Heathrow - both are nice, but Delta has a lot resting on Gatwick. However, maybe they will shift one of their daily Atlanta bound flights to Heathrow, (eg. DL11/DL12) and maybe replace it with either a New York JFK or Salt Lake City direct service from Gatwick? Just a thought.
In response to DL/AF Concorde services from Paris CDG to Atlanta, this looks unlikely as the Concorde would have to be less than full to fly the 4,225 miles. (When full, Concorde's maximum range is only around 3,950 miles) Besides, the route would have to take them out over the Atlanta from Savannah onwards, rather than at Gander / New York etc. as otherwize the airliner would not be able to go Supersonic over land - and what is the point of a supersonic airliner if it cannot fly supersonically?
In response to Virgin operating Concorde from LHR - JFK - that would be seriously cool as Delta would have access to LHR that way if nothing else and would be the only airline with access to Concorde services from JFK to both LHR and CDG.
Hope this answers some questions.