I just finished LHR-DXB-LHR run and got the boarding passes for both segments issued at LHR after online checkin. Easiest turnaround in any International city is DXB. You come out of the plane turn left for transit, go through security and you are in departure area IN 5 MINUTES.

I hope the same will be possible for IST as my next leg will be LHR-IST-LHR with a 100 minute transit time. Any posts about experiences for going through transit without clearing immigration are welcome Of course if BA doesnt give me return boarding pass at LHR, I have to get it at the transit desk at IST as mentioned earlier