I took the Eurostar from London to Paris today. Here's what I experienced at St. Pancras:
You scan your ticket to get into the Eurostar area. From there you go through airline type security but not a stringent. Next stop is exit UK where they look at your passport. I don't know why . Then it's French immigration. They want to see your passport and proof of vaccination. (I showed the pass but I have been told they accept the CDC card.) That's it. Except for showing proof of vaccination, everything else is the same.
There were signs up saying that you will be asked for your French Locator form by Eurostar staff.. No one asked for mine
I then boarded the train. No one asked for my ticket.
In Paris, I had to change station (Nord to St. Lazarre.) The line for taxis was long and it took 30 minutes to finally get one. The taxi louts were out and tried to get me. They ran off when the SNCF police arrived.
The train was a Nomad and no one wanted to see my pass. Just my ticket.
Yes, I know I could have taken the metro. I didn't want to.