FYI United will give miles for Eurostar trips. here's a clip from the UAL site (under Mileage plus - other partners -eurostar)
750 miles per paid segment in Premium First Fare , 500 miles per paid segment in First Class Full Fare or Normal First Fare, 250 miles per paid segment in Second Class Full Fare or Weekender Fare.
Call United for credit procedures, I think you have to mail in the used boarding passes/tickets for the credits. Hope you guys saved those old ones.