I do Whitwell (WWL) to Kings Cross )KGX) and back quite a bit. I often find my ticket checked between WWL and Worksop (WRK) (an EMT service). Between WRK and Retford (RET) (A northern service) and then every so often i might get the opportunity to speak out and show a ticket on the service to KGX. "Any unchecked tickets please". Whilst it may be considered wrong, i often ignore them and see if they walk past, which they always do. I DO HAVE a valid ticket of course.
On the return leg sometimes there is a full ticket check between KGX and PBO (Peterboroguh) somewhere, sometimes nothing until further up the line where it is once again a request for unchecked tickets.
no barriers at WWL, WRK, or RET (though i'm connecting anyway) and i usually get to KGX at a time when the barriers are open (at least the ones on the bridge outside the 1st class lounge anyway). I could likely get there and back countless times on a £4.** WWL to RET return.
I have also done Aberdeen (ABD) to KGX with no checks and no closed barriers, in 1st class, with 2 meals, and a couple of beers and a G&T. I have a valid ticket of course, but i must say i have been deliberately not showing it unless specifically asked as this sort of thing interests me.
Rob