All it needs to increase revenue, is for the chancers to know there is a good chance there will be a check. When you can travel from Liverpool to Euston and back with no checks it really is taking the pee.
back in October I went from London to Wick, 8 days, via several stops on the WCML, Bolton, Blackpool, Liverpool, York, Carlilse, Glasgow, Edinburgh, Dundee & Perth, including a day on Merseyrail and a day on SWR south london.
I have a stack of 40 train tickets, and travelled more then 60 trains (admittedly some were only 1 stop).
ticket checks
on board: Wrexham General - Central, TPE Lime street to York, Rail Charter Services HST, Strathspey Railway, Inverness to Wick & vv, and Caledonian sleeper. I just don't recall any others.
I have the fullset of tickets, only 1 of them was actually stamped (tpe from Liverpool).
There really isnt much ticket checking going on, on the network that I saw, I travelled on Avanti, Northern, TPE, Merseyrail, ATW, RCS, Scotrail, Caledonian Sleeper, LNER, Strathspey, SWR, LUL, Southern that week.
Admittedly having a days tickets in hand, meant I didnt pass through many station barriers, and London / Merseyside used daytickets which no one checked at all, but it feels like the system is reliant on barriers.