What about confirmed seats?
ID100's are confirmed seats - or at least they can all potentially be if BA offer any confirmed seats on the flight you want. As I said, even if you have a firm seat, it does not mean 'you will definitely get on whether a full fare paying passenger wants it or not' you are simply right at the top of the list to get on in an otherwise empty seat. The good bit is that if you don't get on the return journey, BA have the same duty of care as to a normal passenger, and pay your accommodation costs. Never had to put that to the test mind, we have never failed to get on when we have a firm seat.
BA have it to a fine art, they know their routes and the popularity/busy periods, they don't give out 'firms' unless they are pretty darn sure the seat will be unsold.