I had always thought that one could not book an empty seat but it seams that one can. A question that is not answered is :-

"New process to book Extra Seats for personal use or cabin baggage

If a customer requires an extra seat for comfort or cabin baggage at the point of reservation, British Airways now requires that the extra seats are reserved together creating an itinerary whereby the passenger and the extra seat are held in the same PNR. The names in the PNR need to be requested using the following format:

1) For an extra seat for personal use the name of the passenger should be entered using the normal name entry with the following format:
2SMITH/AMR/EXST

A SSR EXST for BA must be entered with mandatory free text explaining the reason for the request e.g. Personal Comfort or Broken Leg

2) For an extra seat for cabin baggage the name of the passenger should be entered using the normal name entry with the following format:
2SMITH/AMR/CBBG

A SSR CBBG for BA must be entered with mandatory free text describing the item and its dimensions and weight e.g. Artwork 20 x 30 x 20 cms 5kgs

Please note the CBBG (cabin baggage) should always be used for inanimate objects such as artwork or musical instruments and must always contain the dimensions and weight of the object.

3) If an object requires more than one seat, then the mandatory free text must contain the following:
EXTRA SEAT PLUS 1 X ADDITIONAL EXST FOR PERSONAL COMFORT
4) In cases where the extra seat is required on only one sector of a multi sector itinerary, then only one passenger should be booked in the itinerary and an SSR BULK for BA must be entered with mandatory free text describing the item and its dimensions and weight e.g. Artwork 20 x 30 x 20 cms 5kgs and the SSR should be associated to the correct segment(s).

SSR BULK can also be used if the request for an extra seat was received on an existing booking.

If in doubt as to the number of seats required for a particular item, or for further information, please contact your local BA Office."