Whilst it's true that most stations were still barriered in the 70s, many small stations, especially suburban, small town and rural stations, were only staffed by one person who was booking office clerk, ticket collector and station supervisor all in one.
Where that was the case, the one member of staff would be in the ticket office most of the time, meaning that it was the responsibility of the passenger to ensure that they bought a ticket before boarding a train, and whenever a train arrived, the same member of staff would stand by the exit to examine or collect the tickets of disembarking passengers (or collect their fares if they had not bought a ticket before or during their journey, whether they hadn't allowed enough time to do so or had boarded at a station that was unstaffed or where the ticket office was closed).
In some cases, where non-gangwayed DMUs or EMUs called at unstaffed stations, paying on arrival at your destination was the usual way to pay your fare if there wasn't a Travelling Ticket Inspector on board your train. I seem to recall that this usually happened for passengers boarding at Morden Road or Waddon Marsh on the Wimbledon-West Croydon line when it was a heavy rail line, for example.
Also, London King's Cross and Paddington had one platform at the far left-hand side of the main trainshed that was unbarriered: Platform 1 at Paddington (which remains unbarriered to this day) and Platform 8 at King's Cross. Presumably those platforms were used only by long-distance trains that had on-board Travelling Ticket Inspectors (and by charter trains and Sleeper trains where a member of staff would check all passengers' tickets on boarding).
In about the mid to late '70s the Two Ronnies did a series called "Stop, You're Killing Me", which was set on a farm in a fictional Devon village called Drake's Bottom. In the first episode they travel down there, presumably from London, in a Mark 1 compartment, and shortly after boarding they realise that they haven't bought tickets, so one of them hides in the toilet and pretends to have both their tickets when the inspector comes round, and the inspector knocks on the toilet door and asks "Are you Dunn?" (because the other one says his name is Dunn) and he says no not yet, and they get away with it!
In the next episode you see them alighting from a Southern Region DEMU at a station masquerading as Drakes Halt but I think it's actually Groombridge on the Tunbridge Wells-Eridge line (now the Spa Valley Railway). As they alight and come out of the station they say it's a good job that they got away with it or they would have had to pay full fare.