as many others have said, there is simply no concept of a seat being "yours" unless and until you are fully checked in and have passed conformance at T5
I experienced this myself - Mrs R was due to fly to the US with me in Club (upper deck 747, no less); we had checked in online and were all set to go; an hour before we leave for the airport a family matter came up which required Mrs R to stay home - BA would have believed her to be on the flight right up until she failed to pass through security at T5 at -35 minutes or whatever it is - her seat next to me was still occupied by another passenger when I boarded the plance
BA don't hold the seat free!