Refunding a train fare after you got on the wrong rail network

I thought asking here might be worth a try in case anyone here has had similar experiences and/or knowledge of the system. 

So I had a virtual ticket (which i am not very used to) from Beverley all the way down to London.  It said it was only applicable to Hull Trains and Northern.  It was activated, but using google maps I followed the only route that was apparent to me and ended up on a London North Eastern Rail train to London, where the ticket inspector told me I had to spend another £92 to be on that train... BUT... that I should be able to get a refund from thetrainline.com  

I asked him if he was sure they would be understanding if I called.  He said that it is illegal for me to be charged twice for the same journey, so they ought to be able to refund me.

So naturally when I did call them, they said no, I should have made sure to only stay on the Hull Trains network. 

I can accept this if it's definitely fair, but is it?  Does the ticket price not just go to National Rail?  Do they not make a lot of extra money from dummies like me failing to acknowledge the difference between rail networks? 

Could I not have opted to reroute my journey after the ticket inspector told me of my mistake, given that I only agreed to pay after he had informed me that I would be able to get a refund?  If you innocently get on the wrong train and can't pay, can you not just leave the train or get sent back? 

I feel a bit odd bringing this up here on a forum I've never posted on, but I have been getting the NAS publications since I was diagnosed with Aspergers as a child, and I just wanted to run this situation by someone with insight as to whether this situation is a bureaucracy trap or if I'm fairly paying for my mistake.  I'll be amazed if anyone reads this, so thank if you did :)