Just to be clear, you will buy a separate ticket from somewhere to London, then you hope to travel from London to Brussels(?) and on to Germany using your pass. But you don’t want to activate the pass until you know you will definitely catch the Eurostar?

That’s fine. Good advice not to activate the pass until you will definitely use it. But you’ll have to make a new reservation on Eurostar the next day (if available)

Or are you using your pass in England to travel to London on Day 1 already?

Thanks for your tip. I have a 7-day pass. However, I only need six days. So I can use my sixth day for the journey to London (5 hours) and my seventh day for my journey home to Germany on the same day. Of course, then I have to make the reservation again. It would be the worst case.

If I put my sixth day directly down for the journey home, but miss the last Eurostar, I would have to pay for the full train the day after. A journey home can only be entered once.

If you miss your train because of delays of cancellations, then the railways are obliged to get you to your destination without any extra costs. They will also have to provide overnight accommodation.

Thank you, but that doesn't answer the question. I don't think you get an overnight accommodation offer for, say, 30 minutes late. That is not the point. 

If you enter each train as journey and then only activate the journey just before boarding that train you will have no problem. Your inbound travel day in Germany will then be activated when you board the train that will take you to Germany, no matter what day that will be.