I used to do this on the East Coast line all the time in the 90's when I was in college at Newcastle, I would always have money for a ticket but if no one asked I wouldn't buy one, or I'd buy one for the first part of the journey and stay on till my destination... can't say I ever did or would lose any sleep over it.
If you catch the later trains and the guards can't be arsed doing there jobs then sod them , more beer money for me.
On an aside , If you got the train from Edinburgh to Dunbar it cost about £9 ,if you then bought a ticket from Dunbar to Newcastle it cost around £15 ,however if you bought a ticket from Edinburgh to Newcastle it cost over £30 ,when I pointed this out to a guard he just looked at me and said ...'yeh but you cant do that', but without much conviction ..
I always looked on it as a kind of game.