If you fly United, you may wish to keep this in mind:

Now you have a new choice for earning and redeeming miles when traveling between the U.K. and continental Europe. Take Eurostar—the train with convenient routes between London and Brussels or Paris.

Earn

750 miles per paid segment in Premium First Fare
500 miles per paid segment in Full-Fare First Class
250 miles per paid segment in Full-Fare Second Class
You can also redeem your Mileage Plus miles for an award ticket on Eurostar, starting at just 10,000 miles for a Standard Class ticket with a Saturday night stay, through 30,000 miles for a luxurious crossing in Premium First Fare.

For more details, call United Reservations.