I understand that BA blocks the first rows of the Y-cabin in short hauls from pre-reserved selection, so that they can flexibly expand the J-cabin without having to reassign (and disappoint) people in the Y-cabin. Nobody, including emeralds, can choose those seats.

I know that at a certain point (before T-24) this block is removed, and the front of the cabin opens up for assignement. I believe it's around 3 days prior.

Does anyone know what the magic timing is? Thanks.