Not quite the same as you but I booked a basic economy to BKK and joined flying blue at the same time around 1 month ago (I'm BA Silver), so it was a bear bones flight no hold bag no free seat selection just 12kgs carry on. Then I fell over the Status Match program so decided to enjoy a status match. And was matched to Flying Blue Gold. And now I have seat free selection and 1 hold bag. So as suggested you will probably have seat selection post booking.
John