Seat assignments aren't guaranteed - even if you pay, the most you'll get is a refund.

Operationally, it may be easier for BA to handle a single business cabin rather than providing a business service in J and F - in which case, they are quite likely to move you on a quiet flight. The risk then is that you may not be seated together, although you'd have a reasonable chance of swapping with a solo traveller who happens to have chosen the seat next to you.