Well, it's interesting to note that my experience at LHR T3 Gate 22 for AA 39 LHR-MIA on Saturday had no regulation when it came to boarding by group. We were herded into the smallish waiting area after they scanned our boarding pass. A muffled voice announced "boarding first class" followed by business class. Everyone, I mean everyone, just headed to the jetway door. I was BC and the guy in front of me was group 7. So if that's pretty standard for LHR boarding, I can say it's every person for themselves. Mind you, it doesn't really bother me because we all ended up on the plane.