I think DL may park at the South terminal remote satellite (not 100% sure but NW and US do) and if so, you'll have to take an automated shuttle to the main South terminal before going to the North terminal that BA uses. I am not sure where the transfer point to the North terminal is so you don't have to go through U.K. entry (the terminals are connected by a landside non-sterile shuttle).
Is there no ATL-CDG option?