London to Amsterdam Eurostar or Dutch-flyer overnight ferry

Bear in mind that the rail link between Hoek van Holland and Rotterdam is no longer in service, and will be reactivated sometime later this year, after reconstruction, as a light rail line that will require one (after mid-2018) or two (until mid-2018) additional transfers to Rotterdam Centraal. That means you will have to use cramped buses to move from the ferry terminal.

Look at flying out of London City Airport for a fare (or fair) comparison.

I'll be taking the Dutch Flyer in August. Here's a link to a post I made with information on the replacement bus service between Hoek van Holland and Rotterdam -- https://community.ricksteves.com/travel-forum/transportation/hoek-van-holland-rotterdam-replacement-buses

Thank you all for your kindly suggestions and advice. I learned a lot from reading everyone's reply. I have booked an Eurostar ticket from London to Amsterdam with one stop for 59 euro, plus an insurance for cancellation. Next decision will be next leg in the trip: Amsterdam - Brussels or Bruges; and then Brussels or Bruges -Paris. I don't know which one better as far as more convenient from which town to Paris. Can I stay in Bruges for two nights which take a day trip in Brussels? or do I need to spend one night in each city?

If you are planning a one-day trip to Brussel, then you can travel from Amsterdam to Brugge, stay 2 nights there. Depart very early from Brugge to Bruxelles-Midi. Store your luggage in the lockers, and spend the day in the city. Take the last Thalys high-speed train in the evening to Paris (it is just a 1h30 trip). That way ou avoid backtracking Brugge-Bruxelles, as the train route between Brugge and Paris goes through Bruxelles Midi anyway.

Make sure to buy Amsterdam-Brugge and Bruxelles Midi-Paris Nord tickets in advance.

Hi Andre,
Thank you so much for your very useful suggestions. Come to think of it that how wonderful and amazing that people in different parts of the world are helping me to plan my first solo trip to Europe. I felt bit lost and overwhelmed to discern so much information reading books and internet forums, and felt great when you and other people can give me practical info. for my specific situation.
One question about booking the train tickets online, there are seem different stations in parenthesis after the city name, say London (GB), or London St Pancreas or Amsterdam (NL) and (NI), What do they mean? which one should I choose to buy the ticket?

if you are getting country abbreviations (GB=UK, NL=Netherlands, etc) that sounds like a third party reseller like RailEurope. If so, run away. Now.

Use proper national rail company websites. For the ferry book it at Stena Lines or at a British rail company. Don't use off the wall websites.