You can fly or take the train, and I think I’d opt for the train. It should be less hassle and might actually be faster taking into account the time associated with getting to/from airports, check in lines, airport security, etc. It’s a little over five hours, with a change in Brussels. Tickets for your travel date go on sale March 24, and you’ll want to book as soon as possible to take advantage of advanced purchase specials:
http://www.eurostar….

I agree, train will be more comfortable and nearly as fast as the plane. However, this is one that’s pretty much a toss-up so I would fly, if the total cost of the flight (ticket, taxes, getting to/from airport, baggage fees, etc.) ended up being significantly cheaper. The easy access to Schipol Airport from Amsterdam makes Amsterdam an easier place to fly out of than most. On the flip side, depending on which airport, you can spend an awful lot of time and money getting to London from the airport.

By train, you could work in a quick visit to Brussels as Market Square is an easy walk from Central station.

Easyjet looks like $40 total one-way currently offered for that date. From airports to London, check out http://www.easybus.c…

Train: Thalys to Eurostar, fastest currently looks like 4 hrs, 47 mins to St. Pancreas … or uh, Pancras. Timetables: bahn.de. NS.nl the Dutch Rail site, shows durations a bit longer, and tickets around that date starting at 50 EUR.

This one really is a toss up, in my opinion. If I could get flights to LCY for $40, then I’d do that, but those usually are $100+