On a maximum of 2 travel days (00.00-23.59 CET) you have the possibility to also travel in your country of residence. These days are included in your total number of travel days. 

Assuming you have a global pass, you can use your outbounds for as many trips as you want in one day, so all the way from Edinburgh to Paris. 

The outbound trip counts as a travel day. It is in fact a special travel day that you can (but don't have to) use in your home country.

Note one country passes only cover travel within the country you chose, so not the travel to/from that country.

On two of your (probably 7) travel days, you can also use trains in your country of residence. The UK in this case. The inbound/outbound days are not extra.

Using a travel day, consumes a travel day.

On a travel day (0:00 - 23:59 CET, or 23:00-22:59 GMT), you can board as many trains as you want (so also in the UK, in your case).

You can use 1 travel day to go out and one 1 travel day to get in, two to go out, or two to get in. Most people choose the inbound/outbound travel days for the most expensive journeys otherwise.

Eurostar always uses an inbound travel day to/from the UK as it is your Country of Residence.

Thanks - still trying to establish for sure whether these can be multi-leg journeys as long as I end up at an exit point, or only single legs, if anyone can clarify for sure. Extreme example - if I were in North Wales somewhere, could my outbound travel day be one leg to Manchester, a second to London, and then a third to Gatwick?  Or would only the London to Gatwick leg be allowable on my interrail pass?

Thanks..

You can take as many trains as you want on a travel day, so including every leg is possible. (As long as the company is part of the Interrail scheme, but most companies are in the UK).

As many trains as you need to get from your origin to departing the country but note that it’s intended to be a reasonably direct journey.

So you can’t go from North Wales via Scotland or Penzance to London, stopping off for lunch or other appointments on the way!

If you’re travelling on LNER, Avanti or Transpennine, there’s a quirk of the timetable that means Interrail thinks you need reservations. You almost certainly don’t, but since they’re free to make, you may as well do it anyway.

So the Inboud / Outbound days of travel are not included also in the “One Country Pass”?

No they’re not

You cannot travel in your home country at all with a One-country pass. Sorry if I didn’t make that clear.