No guarantees that you will be seated togethr although I would not choose to pay to book seats. It is not a very long flight after all.

Seat selection is free only if you have frequently flyern status like BA Silver or BA Gold.
Even if you wanted row 1, you could not pay to book these. They are normally blocked for BA Gold members until 24 hours before the flight when they are open to anyone if unreserved.

If you wish to find out about pre-booked seats on your particular flight, send me a private message about your dates and I can look these up on expert flyer. You can also post it on this thread openly and various other regulars here can look up the same information.