>>2602 You may be able to get a cheap advance through ticket from your local station, though apparently for some reason you have to get in touch with Eurostar's domestic sales office directly for this.
The alternative is flying from shitty airports in the middle of nowhere (LHR/LGW will understandably be more expensive), and putting up with the kind of shitty airlines that fly from them.
I'm in North London, so I'm pretty close to St Pancras International for Eurostar and Stansted airport for flights. There's more complex travel on the other side to get to 's-Middle-Of-Nowhere, Netherlands.
The train from Brussels to A'dam is only fast if you shell out and get the Fyra train. If you get NS/NS Hispeed you're in for a really long trip on a clunky old train. It's also not much cheaper than getting the plane (can actually often be more). Don't forget that Easyjet fly into Schiphol now.
Cheapest hands down is to get the Eurolines bus from Victoria, but you're looking at a bit of a treck. I've never done it myself so I don't know how long it takes, but I know it's 8 hours A'dam to Paris on the Eurolines so make of that what you will.
Weird. I've used this route dozens of times myself using the Eurostar site and I always get the choice to book Fyra, ICE or NS Hi-Speed for the Brussels to A'dam leg.