Can't speak for BA, but for my airline, we control when the gate details are released and put up on the screens. Tonight, for example, i will probably find out the gate number at around 1800 as the inbound aircraft is due around 1830. But with a departure time of 2120 we will probably not put the gate number on screens until around 2020. Not sure if there is some rule governing this. I know that here in T3 we would have problems with crowds gathering outside gate lounges before opening time and blocking the way for other passengers.