The reservations are separate from you pass, which is the ticket. When making reservations you need to keep track of the outbound/inbound rule yourself. 

The outbound/inbound rule says that you can only travel in your country of residence during a maximum of 2 travel days.

You need to purchase a normal ticket for the part of the journey within UK and the Eurostar for one of the trips and then hope that you can cancel those reservations.

Okay,  but my reservations which I've printed at least for Amsterdam are direct and will allow me onto the Eurostar, and they say holder of a valid pass.  Is there no way that alone would be enough to travel to and from Amsterdam? Or do I also need to show the pass itself? 

You need to show your pass as well and in the pass you need to create a journey for each of your connections, like from your home to London, London to Amsterdam and so on. For each journey you the create a QR code in the app, that is your ticket.

You need both the ticket and the seat reservation to board the Eurostar. The app will allow you to travel on 2 days in the UK, thus you can only use the Eurostar on 2 occasions.

But I’ve been able to use my seat reservation to print out a Eurostar ticket, and it says that is all I need to get through the ticket gates. Is this not the case then?

I have not used the Eurostar, so I don't know, but if it would work and someone would ask for your ticket you can get a heavy fine for not having a ticket.

@seewulf @rvdborgt 

The reservation alone will probably get you through the ticket gate. But you also need a valid Interrail, since your reservation is only valid together with a valid pass.

The reservation says ‘Holder of valid Eurail pass’, seems very strange. Thank you for your help. 

Do you know if the journey can be activated once you are on the train or must it be beforehand?