As part of their 1 year anniversary celebrations of trains linking London to The Netherlands, Eurostar have got a quick flash sale offering one-way tickets along the same route from £35 in Standard Class. You need to book before 15th April 2019 for travel between 6th May – 19th September 2019.
There are a maximum number of 15,000 Standard Class seats on sale for £35, and if you want that extra space there are 5000 Standard Premier seats on sale for £79.
If you have never taken a train between London and Amsterdam this is probably marginally slower than flying depending on how long it takes you to get to an airport or London St. Pancras. But if you have an extra hour to spare and want to take a check in luggage with you, there are few cheaper ways to do so than by train.
Note that on the way back from The Netherlands you need to take a Thalys train and change at Brussels-Midi/Zuid to clear Schengen security and passport control to then take a train back to London. The sale portal page is here.