Originally Posted by KARFA

I have done a b2b at DME about 1.5 years ago. I was able to get a bp on the app for the DME-LHR before starting the LHR-DME flight. If for whatever reason you can't do that you need to speak to BA at LHR and get yourself checked in before you fly to DME. At DME the process of doing the transfer was fairly simple, do not go through immigration but follow the signs for connections/transfers. You go to a separate room and do a security check and a bp & passport check and then come out in to the airside departures area.