I've just booked LGW -> AGP at the start of Sept via BA, and it turns out that these are Vueling flights. Just looking to prebook my seats (on the Vueling website, since there's no ability to do this on MMB), and I can change my seat, but for a fee. It's not clear to me whether I need to pay or whether I can do these FOC as a GCH. I can't see anywhere to enter my BA Exec number, so do I just need to pay, or is there a way of doing this for free? Either is fine, I just don't want to pay £50 for 2 seats if it's not needed.

Thanks in advance.