so much good information and context, much appreciated! Sounds like I'm probably better off sticking to one-world and AVIOS, but it is really frustrating to see that the secondary screening at LHR during connection is so strict and they will literally put you in a long line to check the begs that just passed security at another airport for a BA flight that just landed. Wouldn't they solve also a lot of the long lines issues if they were able to separate passengers arriving from certain airports, for connections? I literally just cleared security in Milan 3 hours prior to my connection at LHR, but then have to go through a long line to scan the very same begs.
What makes it difficult for BA at LHR to keep connecting passengers from "clean" airports on a separate, post-security, lane?