Actually it is the case at Liverpool Street last I knew; in the peak trains are generally only available around 10 minutes before departure, and in most cases the driver is already on board at that time - most trains come off the depot and the driver is not due a break.Rather sweeping generalisation there...And also one that wouldn't apply at most commuter terminals I suspect, due to the very nature of the way the diagrams are worked meaning that the trains aren't generally "ready to depart" much more than five minutes before booked time.
Each station should have an evacuation plan detailing what to do in the event of needing to detrain passengers during an evacuation.
And if the controlling signalbox is co-located with the station, such as at Marylebone, Liverpool Street and Kings Cross, I'd expect the 'box to be evacuated along with everyone else, so you'd get no more train movements in the controlled area until such time as the signallers were back in place.
But yes, it is a sweeping generalisation.