The Amsterdam/London Express was, for several years, the most hassle-free method of getting between the two cities. One ticket would get you from London Liverpool Street Station via train to Harwich, then on a fast hydrofoil to Hoek van Holland, and then finally on another train to Rotterdam, The Hague or Amsterdam, in around six hours.
Sadly, the hydrofoil was removed at the beginning of 2007, but the service remains, now using ferries, and renamed as the DutchFlyer; possibly in jest, as the service takes twice as long as it did previously.
However, it still remains one of the least problematic methods for getting to the Netherlands, since there’s no annoying airports to deal with. And, as The Guardian explains, you can travel with a clear conscience, too.