Another FT'er asked me about DME by PM. Since I wrote up my experience I thought I would post it here as well in case it is of any use to anyone.
There are some previous posts/threads here:
http://www.flyertalk.com/forum/briti...-time-dme.html
http://www.flyertalk.com/forum/27100728-post1194.html
So the first issue is visas since most people would normally require one for Russia. I had printed out some timatic extracts to help in case I needed them - you can get them off these sites:
http://www.iatatravelcentre.com/GB-U...quirements.htm
https://www.emirates.com/english/bef...uirements.aspx
Basically we were relying on the "transit without visa" provisions which say you do not need a visa if you hold an onward ticket. Now technically a transit should be when travelling from one country to another and not heading back to the origin, but this was never raised by the BA or G4S agents. You should be able to get your DME-LHR bp before going to do your visa check before your LHR-DME flight - I could only get the bp by printing online since mobile bp wasn't available for DME-LHR.
For a gold card or F passenger you can use first check in so I would go there to do your visa check and not to the other visa check desks near north/south security. The BA agent will obviously asked if we had visas but we just said we are making use of the transiting without visa provisions and do not need one, and we have a bp for an onward flight from DME. The agent on Saturday was more confused about why we were doing it tbh.
The BA agent then called a G4S agent to come and check. The G4S agent that came frankly gave a cursory glance and agreed it was fine in about 2 seconds. If you do get a few more questions from the G4S agent then I would suggest sticking to the TWOV line and showing your onward bp.
In terms of DME, when you get there you will go down some stairs after exiting the airbridge, along a corridor, and towards the end you go up an escalator - follow the signs for transit. When you go up the transit door is a bit hidden but is on the far left of the hall - do not go through the manned passport desks. You basically go inside a little room where you go through a metal detector and put belongings through an x-ray machine. The security agent will just want to see your onward bp. Once through there are more security agents who ask to see your onward bp and passport. Unlike previous reports, the one on Saturday did want to see the inbound bp too which obviously prompted some questions about why we were coming from London and heading back. I just said because we wanted to fly and enjoyed flying and that seemed to be enough - she probably had us in the harmless idiots category at this stage!
Once that is done you go though the opaque glass automatic door and you are in to the departures area. The BA lounge is the navigator lounge and you will have to go upstairs for that. This all happens within the same pier so the distances are not large. The whole process from leaving the aircraft to getting back in the departures area took about 7 minutes in total I would say.
EDIT: see this thread for a more recent experience in which BA said no Back to Back in Moscow - Denied Boarding at LHR