Basically, very long flights need more crew/a change of crew, and the crew rest area in the 789nis insufficient to accommodate them (whilst enough to accommodate crew needs on a long haul with just one set of crew). So on those very long flights, 7K is always used as crew rest.

To go back to the OP's original question, no, BA won't give you priority for FLUB because you lost your seat assignments. FLUB will also only be used if there is no other option (ie if the J cabin isn't sufficient to accommodate all confirmed J customers and necessary opups). If so, BA will open FLUB for just the number of passengers it will actually need to sit there and normally in strict order of status.

Of course if the op has a high status and the flight is overbooked, it may mean that they are lucky, but there is simply nothing for you to do that would increase the chances (and no, asking won't make a difference either). The only other relevance of FLUB, especially if not with a high status is that if other passengers are FLUB, their previously assigned seats may well become available again, though to be honest, when flights are a long time away, seats will become available anyway as people cancel, change, etc, so just keep checking regularly if you don't like 13E there is a reasonable chance you'll be able to change for something else at some point or other.