Well that was a waste of time. I called AA who said that because it's a BA codeshare they can't select a seat for me. I then called BA, who could reserve a seat, but only the standard seats. As a One World Elite I should be able to reserve the premium seats (10A is what I was after and which is available).

According to AA I will have to wait til check in (or take one of the seats down the back that BA are able to reserve for me).

sihctr, how did you get round this? Or is what I've been told incorrect?