Thanks to a tip here on FT, I was able to get return tix on the Eurostar for myself and my wife, and throw away the return, much cheaper than one-ways. As we were tourists in London, getting to Waterloo (where the train left at that time) was a snap.

Arriving at the Gare du Nord, a 10-euro cab ride got us to the hotel in short order.

All in all, for tourists or residents of London or one of the cities on the route, the train can be much cheaper, faster overall, and much more pleasant. I'd do it every time.