Using Expedia's seat preview I noticed that seats D-E-F of the last two rows are reserved on every British Airways flight.
I can guess many reasons for that. Does anyone know the answer for sure?
On long haul flights there are often seats reserved for crew rest. One crew works 5 hours or so then changes clothes and goes to those seats to eat, read, gossip and possibly sleep. The other crew, who was in those seats, changes and takes over duties. They use the very back row because it's close to where working crew hang out during the flight anyway.
I've seen this happen on SAA, complete with changing into uniform-like clothing that is more like sweatpants than a typical flight attendant outfit, on an 18 hour flight (with a tech stop.) I've also seen them unpack their own food to eat at that time - the catering on the flight is I guess not for them.
I don't think it's actually every flight, but it could be that for BA, all widebody flights are long haul. Pilots can typically rest in the cockpit, or on some planes there is an upstairs cubby where pilots can rest between shifts.