And when that happens e.g. penalty fares issued without question, people complain that no "discretion" is being shown.
Yes, but when you do start to apply discretion, you then end up with a situation where people are shown it or not based on what grounds? Attitude test? Mates with the guards relative? Looking like a thug and I don't want to tangle with you?
If the expectation is that every time you get caught breaking a rule you'll get the reasonable level of punishment that is designed to go with it, then surely there's less cause for people to complain?
To pick up on the speeding analogy earlier: if you choose to break the speed limit, you expect that if you get caught you will be punished. You hope not to get caught, and you hope if you do you can beg for discretion, but there's no right to it.
Referring back to the Cumbrian Coast - let's imagine you as an RPI jump on the train at St Bees heading south. People get on all the way down, and explain they've been buying off the guard for years. Do you give them the benefit of discretion and tell them next time you'll fine them, and become just one more person in a Northern uniform telling them the same thing, or do you PF them? If you don't PF them and they carry on doing what they were doing, are you coming back day after day after day to check they are now buying on the platform?
This is probably where the yellow card database would be handy - guard fills details in when selling ticket, and the next time that person does the same thing, they're flagged as need to pay a penalty. Heck, you could even program the system to do it automatically - just stick a couple of quid on the fare each time a guard sells them a ticket
Joking aside, there are tech hurdles to it, but it makes sense I'd you want to apply discretion for a first infraction, then move to punitive measures.