Originally Posted by
webup
We cannot do the online checkin exactly 24 hours before the flight as we leave LHR on a SQ flight 25 hours prior to the BA flight so will be in the air!
Why not give a trusted friend your PNR and ask them to OLCI at the appropriate point?
As
G-BOAC's link shows, this is BA's seating policy, so there's not really any way of getting round it. Search on "seating" in the BA board and you'll find it's a long and well debated topic - but not one for which there's an easy or quick workaround in circumstances like yours unfortunately.
One long shot - do you have any status with Qantas?