..........BA couldn't manage to offload one no show in the 40 minute period between gate closure and departure.
No - because we have no way of knowing whether an "On Line Check In" passenger will show at the gate or not. Our Flight was delayed by over four hours so when we decided to offload the four no-shows, there was no time for the lady in question to get through security and down to the gate!
(Unlike America, commercial stand by passengers are held at check-in and not permitted to access the gates until in possession of a seat number.)