London -> Rotterdam/Amsterdam
The train goes from St Pancras International to Amsterdam Centraal, with a stop at Brussels-Midi/Zuid and in Rotterdam afterwards. It takes 3 hours 13 minutes to get to Rotterdam and 3 hours 55 minutes to get to Amsterdam.
Connecting via Thalys
There is still be the option to book London > Brussels (Eurostar) > Rotterdam/Amsterdam (Thalys) on eurostar.com
The trip to Brussels takes just over two hours. At Brussels-Midi/Zuid you change on to a Thalys train for the remaining 1 hr 49 mins to Amsterdam Centraal.
Please note that Eurostar + Thalys option is not available on GDS Air
Amsterdam/Rotterdam -> London
The train departs from Amsterdam Centraal (the Eurostar terminal is situated on Platform 15b) stop in Rotterdam Centraal (the Eurostar terminal is situated on Platform 2). The journey to London is 4 hours 09 minutes from Amsterdam and 3 hours 29 minutes from Rotterdam.
There are currently four trains from St Pancras International to Rotterdam / Amsterdam Centraal per day: