I just wanted to update the end of my story - since so many people commented and I got such a wide variety of suggestions, We are American citizens who had a train out of Paris to London, then 18 days in London the flight back to the US. We have family in London, and were going to see our new grand daughter for the first time. We had wanted to go to UK first but they had the most stringent entrance requirements and there was the danger of the entire trip being derailed if we flew in to London as usual. So we decided to spend the holidays with the family.
The French police refused to allow us to board our Eurostar train from Paris to London yesterday. They did not care that we were at day 88 of our Shengen trip (out of a max of 90 days). We lost our trip and got no refund from Eurostar at all. I spent over 2 hours in three lines to get a Eurostar from Brussels to London, and from Paris to Brussels. (With the rail pass another 110eu, without the rail pass about 400 eu). We ended up paying 300 eu to fly to London that same night.
A few lessons for other travelers :
The Eurostar app showed all the trains to London booked that Friday Dec 19, but later I was told that the app only shows the seats eligible for the rail pass. Had we pay 250 eu each, we could have gotten out of Paris that Friday. However, it was impossible to know this. When I went to the Eurostar desk upstairs at Gare du Nord, they have screeners who don't let you enter unless you have a valid ticket for imminent departure. This situation is like trains in France and Spain which might have seats on a train, but they are only for paying customers, not for Eurail pass holders.
Eurostar does not give refunds even when the French government causes the problem. They also charge really high prices for same day travel, but much more reasonable for next day.
The French police don't care about you or your problems. We were at day 88 out of 90 days for Shengen. The police said that we had three months (which is incorrect) and said we had 4 days to buy another ticket (and lose those fees too).
The French government is not reasonable or rational. They were essentially forcing us from overstaying our visa, or pay $2000 per person to go back to the US immediately - and lost the chance to see our family. If they are actually concerned about covid, then the sooner we leave their country the better. This seemed to be punitive because UK has not renewed fishing licenses and there is a public spat between Boris Johnson and Macron, so they are limiting anyone from spending tourist dollars in UK under the guise of covid.
Immediately we noticed a much nicer attitude from everyone we met in Belgium - airline ticket window, the police checkpoint the restaurant. Everyone went out of their way to be accommodating in a way I never see in France.