Are you sure it's not available? I saw the £9 fare available on a number of Saturday flights CDG-LHR on
http://www.flybmi.com yesterday.
It's up to DL when they check you in at ATL whether they're willing to check you through a flight on a separate ticket. Since you don't have a paper ticket, I'm afraid I'm not hopeful that they'll be able to verify your reservation on their computer system.