What elite status do you have with AA, BA or IB? Even in J, BA won't let you select seats for free in advance unless you're on a full fare ticket or have certain oneworld elite status. Otherwise, you can pick a seat at check-in. See
https://www.britishairways.com/en-gb...ving-your-seat.
But, you should be able to buy a seat selection if you want at britishairways.com with the BA record locator.