It used to be cheaper to buy Eurostar tickets online on the Eurostar website by choosing “Belgium” instead of “United Kingdom” on the opening page of the site. Evilly, the country choice page sets a cookie to prevent one from going back and comparing prices. (Granted, it is possible by clearing browser cookies, but it’s not easy for an average user, I suspect.)
However, no matter which site one chooses, it’s possible to have the tickets delivered to any country.
The Belgium/UK difference doesn’t seem to be significant any longer—probably it’s not legal under EU law to discriminate—but one can still save money by choosing the US site, according to the Guardian. It’s worth knowing and checking when buying a ticket.