I also flew upstairs on a full fare economy to Chicago (and onward to Phoenix). What I particularly remember is the return flight where we were later back into Chicago (from PHX). As three of us arrived and rushed for the connection, we were the last on and they more or less shout the doors behind us. We inquired if our bags would make it and were assured ‘yes’. I didn’t believe it, but amazingly, there they were when we arrived at LHR!