Inquiring minds want to know, how many Threadzians are affected by this one?
I've personally never done Interrailing but I know a couple of people at the office that have. Not sure as a Brit it'll really effect me (especially as I'm near London), but like Neil said this is more likely to effect Europeans wanting to go further afield than London when they visit the UK; and with Brexit inbound we're already going to be making it harder for them anyway so why not make it even harder again (headdesk).
I can see Edinburgh getting a hit from it, as that's quite touristy and obviously has a direct rail link from London. York perhaps, as there's a lot of heritage there. Not sure about Manchester or Liverpool, as I've actually never visited. There's always a lot of tourists when I visit the Lake District but the majority tend to be from Asian countries, not European (and you really need a car or bus tour to get around there).
Due to where the UK is, I can very much see that it is going to likely be at the "end" of your journey, or you'll likely going to just hit London (as otherwise you have to travel both up and down the country to move on), so perhaps those that would have visited further afield in the country will just go further across in Europe (i.e. into Spain and West France, if they were coming from the East).
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.