Not just GWR▸ infrastructure related chaos.....grim scenes at Euston (people not even being allowed into the station) & Kings X too.........as well as all Eurostar services cancelled due to a strike......not a good day for the railway.
Eurostar is due to French staff going on strike, they do not need to give notice of strike action in France.
Five days notice of a strike is required in the case of government services, and also private companies providing public services. So that does apply to railway operators. However, these are tunnel staff ensuring safety, and I presume they fall outside that definition - or at least the think they do. It's all very complicated and legal.
