Originally Posted by corporate-wage-slave

You have got the process correct, and it's a long standing feature of BA.com (as in 10 years plus). What you need to do in the first round of booking is NOT select seats, but do add in your Sapphire number. Then after payment has been confirmed, and invisibly your Sapphire status too, you should be able to go back in to Manage My Booking to select your seats for free a few minutes later. If this seems rather strange, the reason being is that those with BAEC status would / should log in first - and thus confirm their credentials - and so they go straight to seat selection on revenue bookings (but oddly, not redemptions); whereas you don't have a mechanism to log in.