I found this information (miles earned in EuroStar and award travel on EuroStar) in the Mileage Plus Member's Guide:

http://www.ual.com/files/15099DOM.pdf

accidently, of course! I was seriously considering using 20 k miles for a trip to Paris. Getting to Waterloo isn't the easiest thing (from Heathrow, that is) so I won't be using it in place of a flight to Paris. More of a lark, really.