the rule of the country of residence exist to avoid people buying an InterRail ticket to use it as a let's say ticket for everyday. To use it to go to work everyday for example.
The idea of InterRail is to travel during some days a country. If I life for example in France (=country of residence) and I need to go to work every day from Paris to Marseilles, an InterRail ticket would be a cheap solution - but this is not the idea of InterRail. It is not the idea to get a cheap ticket to use it as a normal ticket - there are more expensive solutions available
and to avoid all this, the rule of the country of residence exists.
I understand this rule - but I really don't like it. it is not fair! and for example for people living in Germany, the InterRail ticket gets unattractive. Because it is not possible to travel from Southern to Northern Europe without paying supplements.
People need to use a cheap flight (ryanair, ...) - and that is not really ecological, but if you do not have money, you need to do it. And when starting a travel already by plane, you do not need a train anymore. Take the plane, fly to everywhere in the world, it is sometimes even cheaper.
Also it is simply sad, that you can visit the whole Europe, but not your own country.
I know for example really a lot of cities and places of Europe, but only some big cities of my own country... a bit strange. isn't it...?!?
okay, now the rule is explained...
and to help you ... you have to proof by official documents, that you lived in a certain country for more than 6 month that it becomes you country of residence.
and you are living for example in the UK for a long time with you Spanish passport. normally you have to buy an UK-InterRail-pass and you couldn't travel for free in the UK. but until now, no one working at a station could explain me, how to proof this... and what they mean by official documents. if you are coming from outside of Europe, you have such official documents with your visa.
now back to you. if you do not have official documents to proof that you lived in the UK for at least the last six month, you can not buy an UK-InterRail ticket, you have to buy a Spanish ticket which allows you free travel in the UK.
For your friend: he has to buy an UK ticket.
I hope I answered your question a little bit ...?!?
Have a lot of fun... and a lot of sunny days ... and by the way: for Scotland it is nicer to have a car with you ...!!
to reach all the nice and sweet places in nature, you can't reach them by train. I already tried it!