The problem with free rides is there is always one. Normaly someone who is new, or newish and have just come out of training.
I have a policy, but wont go into it here, but politeness goes a long way. The only time I have ever been miffed off with another rail staff member is finding a northern pass holder sitting down on a packed rush hour coast bound service from Victoria.
When I'm off my patch, I do pay, but since the current priv discount is crap, its normally for short trips, eg metro centre to newcastle, alnmouth to newcastle and either pay before I board, or seek the guard. Mind, once I got to the barriers at a certain large northern/east coast station and found I'd missplaced my tickets, as I was looking for them, I showed my southern pass, barrier was opened.
But back to the original topic. Looking forward to this coming in. The anytime fare from London to Newcastle is just silly and a priv discount off silly is still not worth it, so just drive. A decent fare might even make it worth it to train it up there and then rent a car for visting everyone that I need to when I'm up there.