A long time ago there was some news that Finnair had installed a system for "optimal seating" (I think Amadeus Altéa). This is a system that very well can keep track of e.g. platinums and keep seat next to them free as long as possible, among many other airline configurable rules like keeping family members together even when they did not pay for seat selection. So this would correspond to the "theoretical seat" OP mentions for BA.

The very varied experience among platinums at that time showed that either was not all platinums created equal or Finnair had configured it with a set of rules that made this seat block fail on factors incomprihensible to the member. I assume they still use this, but have no idea how well the seat block works IRL today.