Will you have bags to check? I've never tried, but there might be some USA airport security rules about not being allowed to drop a bag more than X hours before a flight.
OTOH if you can do OLCI and print a couple boarding passes, you could use one to go airside in the morning, use the lounge with your friends, and then leave the airport, returning later to check bags and go airside again (with a different printed boarding pass as TSA document checkers tend to scribble on then, so that the scribble could lead to some questions or an enhanced airport security screening). There might be a landside place to leave bags at ATL, but I suspect it's not in the international terminal.
SORRY....I just looked again at your post after being suspicious about the BA flight (codeshare on AA?) from ATL to MIA and I now suspect that you're asking about a lounge at LHR not ATL, where IIRC BA uses The Club airside in F as its contract lounge, which obviously would not have a CCR.