It puzzles me why anyone would go around the BA Avios website looking for things that do not matter... and wondering where they went wrong.
To get your seat selection, you need to call Sri Lankan Airlines - NOT British Airways!
http://www.srilankan.com/enuk/flying...tact-us-revamp
Tel : 020 8538 2000
You don't need your UL PNR. DO NOT GIVE THEM THE AVIOS REFERENCE >
Just call, and tell them that you'd like to do seat selection for your flight :
1. Flight number, date, from where to where
2. Your surname, and first name as booked.
Voila. Case solved.