Note, that you must travel on your concession to NYC over the same period, not operate as crew, otherwise they will not retain the same onload/upgrade priority.
There is no such thing as a 'confirmed' ticket in BA, they are now known as 'bookable'. Consider them ex LHR to be just a higher priority standby and you won't go far wrong. They frequently, after being made 'ok' revert to standby on the day. Remeber the system-wide load factors for the last quarter where 83% or so - that means the aircraft a virtually all chokka right know - a low priority ticket holder trying to get to a popular destination should consider other means than Staff Travel unless they have a means of access to jumpseats.