I would have thought that in any competent company; both would be near the top of their manifestly forseeable operations risk assessment list with detailed mitigation planned that was slightly more comprehensive than handing out dubious leaflets, shouting wibble, putting their underpants on their head and sticking pencils up their nostrils.
Said scenarios being:
1) Channel Tunnel Shut.
2a) Line blocked between Ebbsfleet and Stratford.
(2b and 2c being line blocked between Ashford and Ebbsfleet and line blocked between Stratford and St Pancras).
All TOCs have a list of contingency plans (agreed with mutual operators and NR) for when infrastructure is not available.
Eurostar is not the same in this regard. Scenario 1 would effectively be pointless seeing as nothing can depart St Pancras. Other scenarios might involve trains turning short, but again there is the whole passport control / boarding element to consider which no other operator has. I suggest this would take (considerably) more than 24h to get into motion, if indeed it is even possible post-Brexit.
You are then limited to utilising alternative transport and again Eurostar is not the same; there is no 'diversionary route' or alternative rail ticket acceptance. So you then have to reach out to airlines / ferry companies which is completely different and hope there is availability - one trainload is maybe 6-7 planes. Or allow people to take hotels. Which is broadly what the "dubious leaflet" suggests.