ATW seem happy to sell tickets on board - no guard has questioned anyone, and I saw a lot of it when I had an annual season. There may be a notice to that effect on Llanelli station, I don't know I haven't looked at the enormous number of notices on the ticket office window, which in any case can't be read once the station is locked up at about 3pm!
I think there's a difference between byelaws and laws. There are many byelaws (on and off the railway) that people inadvertently break on a daily basis. These are not usually enforced, it's often enough for an official to have a quiet word, point out that a byelaw has been breached, and ask the person to stop doing whatever it is they are doing. If they refuse, then fixed penalties, arrest and prosecution could occur. I was trained in assertion and conflict avoidance by a police officer who basically said that the first stage was a polite request, second was a polite explanation of the offence being committed and another request, thrid stage was a warning of the consequences of non compliance along with another (less polite) request, and then a final plea before arrest or physical removal.
So I don't necessarily think that the byelaw should be removed, but the automatic assumption that anyone who sets foot on board a train without a ticket is a criminal and needs to be penalised, which seems to be endemic amongst some in the industry, needs to be reconsidered.
Which we now seem to have!
I do think there's a difference between running late and deliberately not buying a ticket, and arriving in plenty of time and being unable to buy one (due to a complicated transaction at a one person station and a faulty TVM). Even so, unless it is Virgin's stated policy otherwise, an undiscounted return should be sold on train. Is it Virgin's published policy not to sell return tickets on its trains?