I genuinely feel for you as it happened to me although that was years ago. I ended upstairs in the 747 and 2 family members were downstairs.

I asked onboard and the excellent CSD sorted it out.

If nothing changes then ask at checkin, at the gate or ask the crew onboard, fingers crossed.

Technically BA have this in their terms and conditions

What happens when we change your seat?

Sometimes we may need to change your seat, even after you have boarded the aircraft. Whilst we try to prevent this, it might be unavoidable for a number of reasons including a change of aircraft due to operational, safety or security reasons, so we cannot guarantee seating.

If we have to change your seat, we will attempt to find you a suitable alternative. We will try to seat your group together, then match your choice of window, middle or aisle seat, if possible. If you have paid for an exit row seat, we will try to offer you another exit row seat.

If we are unable to offer you a suitable alternative for seating you paid for, you can apply for a refund. See the paid seating terms and conditions below.

https://www.britishairways.com/conte...mation/seating