Hi,

you need an Interrail Global Pass.

Keep in mind that you can use it only 2 days in your home country.

For Eurostar you can reserve for 30€ in standard class to Paris or Brussel. 38€ to Amsterdam. Seats for pass holder are under a quota. Not all seats are bookable for pass holder. Check if there are seats left via https://www.b-europe.com/EN/Booking/Pass

ICE trains of DB have only optional reservations, you don't have to but you can. Book directly via bahn.com for 4,50€/person (you can change trains and it cost still 4,50€ for the trip).

there is limited amount of seats on each trains for people with a pass

Thanks for all the Help. Unfortunately Eurostar does not offer any interaill seats on the days I'm travelling.

If you're also looking at normal fares, then also check Berlin-London all through DB. They sell connecting Eurostar tickets often for less than via Eurostar directly.

There are very often no seats from Amsterdam (due to capacity restrictions) but more from Brussels. So try from Brussels if you didn't already. It's not much longer to do Berlin - Cologne - Brussels - London

But it may be sold out everywhere already

A quick check from 14 April suggests quite a few choices for Brussels, early and late trains in particular. Amsterdam has some availability on early trains between 0600 and 0800 on most days.

When you search on the B-Europe site link earlier you simply keep clicking more trains to show available trains for pass holders.

I am sure by now you have got the answers to your questions but just in case - 

An Interrail Global Pass is available in 2 forms - a flexi pass which allows you to travel on most of the trains in Europe for a fixed number of days in a specified period, The least is a 4 days in a month costing £232 or £289 2nd/1st class.

The second is a continuous ticket for unlimited travel on all days in a set period. e.g. 15 days continuous use for £519. Much cheaper if you are doing a European Binge Tour but not I suspect for you.

This allows you to board any train operated by participating companies across Europe scheduled to leave in the period from 0000 to 2359 (even if you do not leave your final train until after midnight).

Some trains have mandatory fixed reservation fees and others have optional reservations - such as the ICE trains in Germany.

So you could travel to Berlin from an early departure in London to Brussels for 30 euro each way and then via ICE trains to Berlin, do a couple of days travelling in Germany, back to Brussels and E* home.

It would cost you about £320 return for train fares (or £400 is first class (Standard Premier in E*). You cannot upgrade further than Std Prem.