I've done the london->ams->london route and its very relaxing compared to flying.
http://www.nshispeed.nl/en/ is the best price for tickets, even beating the eurostars own site and it is the same trains. You can select the routes either from or to holland and they don't care. Thalys is worth the extra on the Brussels to amsterdam route avoiding stopping at all the stations in between, more expensive but also has wifi. Paris however would be worth the extra stay compared to brussels unless you like a lot of TinTin museums.