I was in F for the LHR-JNB flight in May, and to be honest, group 1 for boarding was hardly any different to the other groups; it was just a large crowd, milling around. I'm sure we were called together with group 2, so didn't really see the point. It didn't particularly bother me as I knew I had more than enough space onboard for my cabin bag, but I did wonder what the advantage is to priority boarding from LHR with BA?