In so much as a slot can 'belong' to an airline, then the slot still belongs to BA. It relates to the aircraft movements at LHR, not BGI (which is hardly slot constrained). Given that BAs board have stated that acquring slots at LHR is a strategic objective, it is highly unlikely that they would relinquish that particular slot simply because of the retirement of Concorde. The winter schedule starts today and I suspect there will be an additional service already scheduled to utilise that slot.