Hi, I am afraid you have overlooked 1 major point: it is NOT 2 TRIps, it is 2 trips on ONLY MAX 2 days-the mobile pass in fact allows you 2 days (the old style paper was more stringent in fact-a direct trip OUT and later INbound-also on 1 day each).

€* certainly wants you to have a valid global pass to be able to use its lower pass-only fare supplment.

As the trips are still some time away, you have to deactivate trips and plan accordingly.

You do not have to activate passdays so far advance-it is in fact much better to do it as late as possible-preferably on the day istelf, after a check there are NO strikes, mishaps or other assorted failures that happen so often on the rails.

It is perfectly possible to make the REServ needed without activating the pass.

But surely the journey from St Pancras can’t be counted as a journey from home? Does that mean that I’ve already used up my *Eurostar* journeys? This is frankly catastrophic; we took advantage of an offer for senior passes because we go frequently between Leeds and Amsterdam, and flying is dreadful at the moment, but now I’m feeling very stupid. I’m fine buying a ticket Leeds-London. What on earth do we do? Given that you go through passport control at St Pancras, how can Eurostar be treated as a journey inside the UK?? Does an ‘out’ trip end *in the first foreign country*? We can’t just go away for 3 months travelling - I have work engagements in the UK! (That’s why I’m travelling on 9 June as in the screenshot). Explanations for a 10-year-old please...

As you have between London and the Channel Tunnel few kilometers you are still in the UK and because of that you need a valid Travelday for UK It doesnt matter where you go through the passport Control as this have nothing to do with the tarif borders of the railcompanies.
Tarifborder points are usually just a point on the track :)
In rare cases like Passau or Salzburg between DB(Germany) and ÖBB (Austria) it are real stations.

For your example you would need a Ticket from London too the bordertarifpoint with France (in my experience Eurostar doesn´t sell such tickets and you will be forced to buy a normal ticket)

You can use your Interrail Global Pass in your own country during two travel days of your choice for one outbound trip (leaving your own country) and one inbound trip (entering your own country). A travel day is considered to be a 24-hour period that lasts from midnight to midnight. 

You can take as many trains as necessary to complete these journeys, but you are not allowed to make any extra trips within your own country. You can also use them in the order you prefer (that is, you may use the inbound trip before using the outbound one if that better suits your travel plans). You can read this page for more details,