I can tell you the following about the S/O selection process
It lasted one day and took place at Heatrow airport (BA training center)
round 90 minutes interview (Standard questions about your motivations and some technical questions).
Then basic handling sim ride (20-30 min) in BOAC 1-11. Raw data flying.
No written exam. If well prepared for tech quiz, raw data flying and basic Human ressources questions should be ok
Good book to prepare : "ACE the technical pilot interview" by D Bristow
Hope this might help you.
Good luck
