One issues might be that they won't let you board your BA flight. Call BA and have them make a note in your PNR that you are transiting w/o a visa. I would also print out the page from the Delta website where it says 'no visa required" - see link - this is the same information that check-in staff see on their terminals and it very clear that a visa is not required.

http://www.delta.com/planning_reserv...t_information/