Although DB doesn't run their own trains on the line, if you buy enough in advance, you can buy a "Sparpreis Europa London" ticket from bahn.de, good between an

Although DB doesn't run their own trains on the line, if you buy enough in advance, you can buy a "Sparpreis Europa London" ticket from bahn.de, good between any station in Germany and London, for typically in the range of €59-109 each way. That includes both the Germany-to-Brussels (or Paris) and the Eurostar portions of the trip. Can be a pretty good deal depending on the time/date and where in Germany you're going to/from.