There are NO exceptional circumstances for holding off a possession tonight. 1800 is an iron curtian
For what reason?
Due to possessions booked on which a late running train would delay - Possessions can be held off.
Due to Signallers all walking off the job at 1800 regardless of any trains still running - Possible I suppose but very, very unlikely (and outrageous if it did happen, leaving trains, passengers and Traincrew stranded)
Due to Signallers leaving once all trains had terminated but before possessions could be granted - Definitely possible, but I would have expected Network Rail to have a plan in place for such a contingency, and delays could of course happen with trains already en route and with no suitable location to terminate short.
Personally I believe that the last train on Christmas Eve is one, if not the only, exceptional circumstance in which rail staff could have worked together to get people home, despite the undoubtedly desperate state of industrial relations on the railway. It might also have been an opportunity for positive publicity for the RMT and ASLEF, showing how they (and not the management) bent over backwards for passengers.