Almost every staffed station in the UK now has ticket barriers, to ensure that you can't travel without a ticket. On many lines here in East Anglia a lot of the stations are unstaffed but there's a conductor who pounces on passengers within moments of them boarding, to check that they've got tickets or to sell them. Further, there are regular patrols from revenue officers, accompanied by British Transport Police, who operate a strict policy of taking court action against anyone who tries to cheat the system by travelling beyond the station on their ticket. (A couple of months ago I saw someone arrested and taken off the train in handcuffs for doing exactly that).
When I was 'trade plating', about 10 years ago, there was actually a photocopied handbook in circulation, entitled "A Plater's Guide to Train Jumping", which listed the easiest ways to avoid paying fares. (I never used it. On several occasions I actually walked the full length of a train, looking for a conductor, in order that I could pay the fare). I very much doubt that such a document still exists because nearly all of the loopholes in the system have now been closed.
Chris