"Eurostar needs the 75 minute check in because it is an international train."
No, that is not the reason. There are lots of international trains. Eurostar is literally the only one that requires advance check in.
The reason it is like that is that when Eurostar was set up railways were considered old fashioned, and on the way out by the then UK government, so they decided to make it more air travel like. So they added this check in and the security theatre. Whowever had the bright idea that adding the inconveniences of air travel to a train service should have some sense knocked in to them.
Brexit then added more complexities.
Officially it is still 30 minutes. And that is when I still plan to turn up.