Indeed, but all the chat I remember just has to do with MRU-LHR. I haven't heard anyone saying JNB-LHR was a problem, for example. But this thread makes it seem like BA's computer might offer limited A or D availability to an MRU-originated ticket, system-wide.

Not that that's particularly a crisis if BA is the only one playing that game - they tend to have decent competition on many of their "interesting" long-hauls. But again it sounds like the entire industry is moving in that direction, from the above posts.