I would also add that the 7 day mark is important as many bronze status holder can select seats for free. It also a mark for many business travellers as they typically going to book flights around that mark. Could be also that recently BA removed a flight from that route and auto rebooked everyone on your flight so it can be full or even overbooked.
If I were in your shoe I would check time to time. (no more than once a day) to see if anything changes. I would also check with expertflyer the seat map as it can show seats which are blocked.
72 hours from the flight is when theoretical seating algorithm takes over and will show available seats to passengers. 24 hours from the flight I would check in if you can (if there is no seats avail then you won't be able to checkin online) Also you mentioned this is a connecting flight so when your first legs open for checkin you could try to checkin for this flight as well.
Worst case it will be sorted out at the airport as BA can upgrade/downgrade passengers and any last minute cancellations, etc.
I wouldn't be worried too much in 90% time will sort it out sooner or later.