I'm booked SFO-LHR in J class on a BA code share, that is, an AA issued ticket with an AA flight number, but on BA metal. I know that BA charges for advanced seat selection, even in J. But if I'm willing to pay, will BA advance assign seats for me, even though it's an AA ticket and an AA flight number? Any wild guess as to how much the seat selection fee might be? It's not at all clear from the BA website.