Could I use American Express Reward Points to book my Eurostar from London to Paris?
It depends entirely on what country you hold your American Express card in
If you have a UK card, you have three different options! Via Avios, to Eurostar frequent traveller points, or direct bookings. It's a complicated enough situation that I ended up writing a series of blog posts on it. Depending how far in advance you book, a standard class return will vary between 9,000 and 15,000 UK MR points.
For other countries, it all depends. Some country's Amex MR points support one of those schemes, some none, some all. Some of those will be at the same rates, some different.
There is no universal answer, sorry.
It depends on your country. Looking at the US membership rewards catalog, the Travel category allows bookings on flights and cruises, while the Car & Rail category includes only rental cars and Amtrak (US rail) gift certificate.
Looking at the French membership rewards catalog, you can get a round-trip (no one-way possible) with 19000 points
So you can browse the catalog and try to book a Eurostar ticket with your points.