Originally Posted by corporate-wage-slave

Yes, well I suspect we should apologise since this frankly minor issue would be somewhat obscure outside a fairly narrow range of BA travellers (which admittedly includes myself).
This doorway connects the Concorde Room - the top BA lounge at LHR for First travellers and the top tier status passengers - to an area next to South Security. It was closed a few weeks back as part of the work that led to the opening of the First Wing, a specially branded and dedicated security route from landside to the First Lounge and CCR. However closing that door meant that for some CCR users they had to walk a little further than before in certain circumstances. Typically those connecting airside, domestic connections and some shorthaul service flights. We are talking under 5 minutes here.
The reason it is called the Millionaires' door has been confused over time, it's should be called the Millionaire door, since BA allegedly had to pay the airport owners a huge sum to account for the door to be put there, due to the loss of shopping income that the direct route caused. The door was closed as part of a quid pro quo for a similar agreement with the airport owner regarding the First Wing, the exact details of which will probably never reach public circulation.