Recent AR to LAX on BA0268 then BA0283. Group boarding was implemented both ways, i.e. even at LHR.
Both times it was a very positive experience at LHR in particular where lanes were clearly marked for each group (with group 4-5 together). I was group 3 as I am Bronze/Ruby.
No clogging the lanes, boarding was organized, at least until I boarded myself, and fast both times. if someone tried it on that got away with it as I didn't seen anyone being turned back.