Do you mean you showed the QR code of the pass? In that case, the ticket inspector was wrong. All British companies accept Interrail, also the mobile pass. If they can't read the QR code, then they should scroll down and look at the readable information.

The inspector was not so nice because he is seemingly not familiar with Interrail/Eurail passes, which are certainly valid on these train services, as well as any other train service in the UK. Did you explain you were using an Interrail/Eurail pass? In my experience, that usually solves the problem.

Yes! I explained it was a Eurail pass, he looked a bit confused! And he scrolled down to the readable info, but he was looking for a number he didn’t find. 
Anyway, if it was his mistake hopefuly doesn’t happen again. Thank you! 

Happens from time to time. There are so many offers in the different countries that the conductors maybe forget one or two. I recommend to insist it´s a railpass and you thought the pass is valid. Sometimes they call their Control Centre to verify that Interrailpass are accpeted.

As iam takeing quite small routes (“Branch lines”) i had that problem severaltimes but till yet was always lucky to manage it.

Thank you!!