I changed me and Mrs HKPAX seat allocation to reserve window and aisle seats for the two of us leaving the middle seat empty, i.e. so no-one would choose it given better options. This worked, and in a nearly full plane we had the space between us empty. I wonder if this was allowed by BA's system because we have different family names?
I wouldn't rely on it. More likely the flight was not busy and the other passengers doing an online checkin did not want to risk the possibility sitting inbetween two vastly overweight people for X hours.
For example, I've tried the same trick on shorthaul sectors on BA, but it rarely works as cattle class is generally crammed full anyway on given dates/times.
(P.S. Just to make clear, not calling you overweight, just using the term to paint a picture of what others may be thinking when they look at an online checkin screen that has a spare seat inbetween two others)