Two years ago, I arrived at Southampton at 11am to board QM2 with an actual check in time of 2pm. I was on board by midday. Last year, I tried the same with QE2 - check in time 2pm, boarding time 2pm. This year my check in at Brooklyn Cruise Terminal was 1pm and I arrived at 11.20am. Boarded at ten minutes to twelve. Two out of three suggests Cunard will board people as they arrive. I think, though, that grill and platinum passengers get priority and will be boarded as they arrive.