At the very least? I'd expect the failure to serve queuing customers within the time stipulated in a Company's 'Passenger's Charter' to be a lot more persuasive than that! Where the Prosecution turned on the failure to buy before travel, I would expect the Defence to turn on that failure.
It is unfortunate that the TOCs provide so much information that can be misleading to the average passenger.
For example, the EMT ticket office at St Pancras has a large poster outside saying, in large print, "THERE WAS A QUEUE", with directly below that, "NO TICKET, NO EXCUSE. You must always buy a ticket before boarding".
I appreciate that "there was a queue" is one of the most frequent excuses trotted out by fare evaders, but that particular ticket office regularly has queues of almost 30 minutes (as I once had to endure after a TVM failed, with the problem, in the event, still not being resolved!).
Passenger Focus have a helpful webpage on
what to do if the ticket office queue is too long (namely, speak to the guard before boarding), and I agree with their advice, but it is unfortunate that some TOCs (such as EMT) publish overly strict information (implying that one is, in theory, obliged to wait many hours if the queue happened to be that long) whilst others (such as Virgin) publish overly lax information (which, whilst theoretically correct, implies that it is always acceptable to walk past ticket purchasing facilities and buy tickets on board).