Had two previous experiences with this
ATL-LGW
All seats were blocked before OLCI, and they were all "taken" when OLCI was completed, although I remember doing it at t-24 so I think they must have been assigned by status. F cabin seemed to be full during the flight, don't know who managed to nab them before or if it was done by status.
LGW-ATL
Coming back we managed to snag 3 F seats at OLCI (Online Check in) and got another one at the airport. F seat, J service. Quite nice
Hope this helps.
Blessings,
fw