BA Experience
I am slightly less confident than jackieofalltrades.
Our two (fairly), recent experiences with BA, both in Business, suggest that, whereas in the old days, when one checked in on-line 24 hours prior to departure, one was 'competing' for seats only with the premium card holders, now with BA encouraging people to pay up front for an allocation at booking, there are far fewer 'spare' seats available for selection.
On BA026 HKG-LHR, the Mehmsahib and I were actually were flying up from Changi to join the flight, and so we were checking in there at around 14h00 for a 23h25 HKG departure. Cathay had a seat allocation for us on the BA sector that was less than ideal, so I took a trip to the BA Business lounge airside, where a very helpful lady was extremely frustrated to find that whoever had edited the flight had then locked it so that she couldn’t do anything for me, except send a message to HKG to say "oi!". Which she most certainly did, as I saw the print-out when we arrived in the lounge at HKG, and asked whether we could be moved. We were eventually sorted at the gate, thank you.
On BA043 LHR-CPT, I went on-line 24 hours before departure to find that we were placed in two aisle seats four rows apart, and again the seating apparently was locked. No joy with getting any change in the lounge, no joy with trying to get it changed at the gate, but eventually, an extremely efficient stewardess re-arranged the cabin so that we (and at least two other couples who also had been spread round the cabin), could sit together. Thank you to her.
If I was a cynic, I'd say that spreading people around when they do not pay up-front for seat allocations is a good way to encourage them to part with their money next time. After all, it’s another revenue stream for the airline at absolutely minimal cost to them. But the reality probably is that so many people have bagged seats weeks ahead that the poor person trying to edit the flight 24 hours or so before departure is left with very little flexibility, and people with the same booking reference get split up.
If you really want to sit together on what may be a busy flight, you either pay up front, or throw yourself on the mercy of the cabin staff and the goodwill of your fellow passengers, some of whom may be asked to move from a seat that they had requested and apparently been given, once you have boarded.