If you buy a Global Pass yes Eurostar is included.
You need a 30€ reservation from Brussels to London or 35€ from Amsterdam/Rotterdam to London (37€/43€ in Standard Premier). You must book as soon as your dates are set: it's a very popular route and there's a passholder quota (unique).
Note that due to works in the security terminal in Amsterdam trains are only doing London -> Amsterdam. In the other direction you must change in Brussels to pass security.
My advice would be to take the hourly IC Direct from the Netherlands to Brussels and then the expensive Eurostar.
Feel free to ask anything.
EDIT: Let me suggest the Hoek van Holland - Harwich Stena Line ferry too. It's a convenient way to cross the North Sea (daytime and overnight crossing). More: https://www.seat61.com/trains-and-routes/london-to-amsterdam-by-ferry.htm