After you make the booking with Avios, login to your reservation on the Latam website and choose your seats. If for any reason you can't do it online on the Latam's website, call them. If Latam says they can't touch the reservation because it was made by another airline, know it is BS and just HUACA. Have your e-ticket numbers ready apart from your reservation number just in case when you call.
Most likely you will be able to do it all on Latam's website though - BA won't be able to help you with this case though - only operating airline.