As I understand it AA Gold (OW Ruby) won't allow you to get early seat reservations on BA without paying for seat reservations to sit together. I think that is specifically a BAEC benefit and BA Bronze (OW Ruby) allows that at T-7 days but happy to be corrected as I'm not 100% sure.

If you OLCI at exactly T-24 I don't think you will have any issues in choosing cost free seats together. If it is a 747 then the twin seats at the rear are great for couples (row 52 especially) and worth paying for. (in my humble opinion and for which I have done in the past). In general terms though these seem to be snapped up before OLCI opens.

The down side is you will be one of the last off the aircraft but if you have a decent layover time at LHR then that isn't really an issue.

Pete