Hi guys,
Thanks for the warm welcome, in spite of my question having such an obvious answer indeed !
I spent so much time recently reading about award flights redemption and people opening Iberia Avios accounts, and so on and so on, that I forgot about the most basic way points are accrued within an alliance
So I guess I'll just book my AA flight anywhere, since it doesn't look like I'll be able to get an upgrade through BA anyway, and I also don't have any BA credit card (I get 1 Avios per £ spent with my HSBC Premier card and feel it's ok enough not to bother with other cards for the time being)
Thanks again !