How does BA allocate the 789 First seats when the First cabin isn't for sale on that route, e.g. LHR-CAI & vv?

It seems rows 1 & 2 are on the Club seat map, blocked but unallocated. I am OW Emerald (AA) but can't assign them, although every other seat is assignable for free.

I assume in advance of checkin at some point they become available or is it only assignable at the airport? Thanks.