Unfortunately, for passengers who do catch the last train from London, there is nowhere to wait whilst the terminal is closed. Despite the nearby McDonald's being H24, that is the drive through only as the seated restaurant itself is closed and locked.
With regards to cat 6 cover with RFF, it is only between certain hours according to the Southend AIP. Anything outside of that is by prior notice.
The same goes for late flights... Ops, Passenger Handling and the Ramp have all got to have staff who agree to stay on. If they don't, then STN it is.