This is absoluetely pants!!

I used to try to use this on UK domestic and never, ever did I get a guaraneteed reservation. Waitlist was all they could do

The terms and conditions say:

"British Airways will do its utmost to ensure Reservation Assurance is delivered. However, we provide no guarantees due to circumstances beyond our control, which may mean we are unable to offer this service."

I think they include the plane being full as circumstances beyond their control. Actually what could they possibly mean by this???