Hello,

yes Interrail has nothing to do with the European Union. Great Britain or better the participating train companies in Great Britain are part of the Interrail Pass. If you live there, you can use a Interrail Global Pass on 2 days to leave or to get back to your home country (Outbound/Inbound Rule). 

Eurail Pass is for people living outside Europe the continent (UK is still part of the continent). 

thanks Angelo ! 

If you plan to travel from/to the UK with the Eurostar my advice is that you check the availability of passholder seats on your planned travel days before you buy your Interrail pass.

There is a limited number of pass holder seats on the Eurostar and on popular departures and during high season those sell out weeks, and sometimes months in advance.

The best place to see the availability of passholder seats and make reservations on the Eurostar is 

https://www.b-europe.com/EN/Booking/Pass#TravelWish

Press "later trains" and eventually you will find the next available connection. 

If you have a mobile pass you need to generate a Pass Cover Number in order to make the reservation at b-europe. You do that here in the PCN generator:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Although the 2 home country days  are called in and out days please note that they are usable at any time during your pass, do not need to include a border crossing by train, can be used in any order and need not be used if, for example, you fly out and back. They are also part of your travel days (not extra ones).

I find them best considered as up to 2 days where they can be used in your own country in exactly the same way as if you were in any other country in the network. 

If you have a mobile pass they are managed by the app and will notify you if you intend a journey from these 2 travel days.

thanks all for comments!

Is that within the T&Cs though? ie unlimited travel rather than one journey to a port?

It can also be to an airport. And it doesn't say you need to take the shortest or fastest route. Just that you can use multiple trains for your journey.