So you plan to take the Eurostar, from London to Amsterdam?

This needs to be a global pass indeed. If you only do one return trip to Amsterdam, a pass is probably a bad deal. Ordinary tickets can be quite cheap, bought in advance.

There’s also a quite firm reservation fee when travelling with Eurostar and Thalys (of  +/- 30 EUR per seat).

When do you plan to travel?

If you are travelling into London you can of course use a Global pass to get there, as well as the Eurostar( As long as all UK travel is done in trains scheduled to depart between 2300 and 2259 UK time).

You are also limited to a max of 2 days travel in the UK, irrespective of the duration of your pass.

Trains from the UK to Amsterdam are limited and there are far more to Brussels, from where you can get a reservation free Intercity service (Avoid the high speed Thalys).