You can use the Interrail fee on the Eurostar on 2 occasions according to the Outbound/Inbound rule.

If you plan well in advance the cheapest normal ticket on the Eurostar is about 40 EUR and the reservation fee for an Interrail pass is 30 EUR in 2nd class, so the difference is not that big.

Thank you Anna.  As you say, the difference isn’t that great, but it’s not what I’d planned for. I’d hoped for a bit more spontaneity.  I’d checked very carefully to be sure we were even allowed to use Interrail now brexit has happened (and you can be sure we didn’t vote for it), but hadn’t spotted this problem of returning home.  

For many years you didn't have any free journeys in your country of residence with Interrail so having 2 free travel days in your home country is good compared to that. 

And even good that Eurostar finally recognise Interrail there were many years that they only recognised Eurail and not Interrail.

As about anywhere with young people on transit: more as half first have to first stuck their nose (=pay more as expected) before they start reading & realising.

Sure it wan't the other way around? I bought Interrail reservations for Eurostar back in 2001 or 2002, albeit for a higher price than now.