My recent LHR to NCL flight was a decent effort for group boarding.
I was group 2, waited until it was called before standing up and advancing.
Most people on lower groups were already lined up and waited patiently but there were several G4 chancers in the G1/2 line who wandered on anyway.
Perhaps it's a judgement from the gate staff. If the numbers in each group look roughly about right, there's little point in intervening. If everyone is piling on in one big line, it might be best to step in.
The BPs are scanned by machine and don't care which group you're in, so if you manage to get in the queue you'll be fine.