I'm going to share my perspective as a former experienced debt adviser. I dealt with County Court claims a lot. I am not a lawyer.
For the calculation of losses, the TOC will argue the contractual agreement states that if a valid ticket is not held then the person is liable for the full undiscounted fare. Clearly the TOCs interpret this to mean the anytime single fare in both directions. There has been some success in the civil courts in arguing that the actual loss to the TOC is the difference between what the person actually paid and what the person would have paid had they bought the correct ticket at the time. However to my knowledge these cases are not legally binding (County Court judgments do not set a legal precedent) and so it would be for both the claimant and the defendant to set out their position and it would depend on the District Judge.
The Courts historically took a dim view of penalty clauses (or clauses that effectively have a punitive element) in contracts but, in more recent times, the Courts have been much more content to enforce penalty clauses unless they are manifestly unreasonable.
The Courts will generally not permit administration costs outside of the fixed fee for bringing a case. If a TOC has incurred specific losses in investigating a case then this can be added to the claim, but generalised costs (e.g. the pro-rated cost of the RPI) would not usually be allowed. This has been established in the Courts whn they (eventually and belatedly) disallowed shops from recovering the cost of security guards from shoplifters. The revenue protection policy is not part of the contractual agreement.
The Courts do expect both parties to correspond transparently before litigating, and I think it is clear that GWR's staff in this case are acting as though they have the same upper hand that the criminal law confers on them.
If you receive a claim pack you only have 14 days to respond to it. You can respond on paper or, if the claim has been issued electronically, you can also respond electronically.
If you need more than 14 days to decide, you can file form N9 'acknowledgement of service', this gives you an additional 14 days in which to file the defence form N9B.
You can file form N9A, which is admission of the claim, in which you can make an offer to repay in instalments. These instalments can be as little as £1 per month. The claimant will decide if they agree to the proposed payment plan, if not then the Court will decide. This will be based on your income and expenditure, and so it is important to be accurate with your offer; don't take the piss and offer a pound a month if you can afford a lot more, but equally don't overset yourself if you have a low income and/or high priority expenditure.
You can file form N9B, in which you set out your defence. On form N9B you would set out how much of the claim you dispute. If you partially admit the claim you would state on form N9B how much you admit. If you are only partially disputing the claim, you would submit form N9A and N9B. The claimant would then decide if they accept your partial submission and, if so, whether they accept your offer to pay in instalments.
Where you send the form depends on the form you are completing, so read the guidance very carefully and keep evidence of the form being returned, especially if sending by post.
If you defend the case in full or if the claimant rejects your partial admission then the claimant has to pay an additional trial fee. They can also claim up to £95 per person for any witnesses for their loss of earnings or leave, although you could challenge this in Court if they are people who would be at work for the claimant anyway. I suspect the Court would agree with the claimant, though.
These types of claims are typically (but not always) heard "judge in chambers", which means that you are in an office with the judge and the claimant's representatives. Not as intimidating as being in open court. I've done it a few times, both as a claimant and as a Mackenzie Friend for a respondent, and it's not that daunting but you do need to be very well prepared.
Finally, even if you lose entirely, then if you pay the entire amount within 28 days then you can apply to the court on form N443 to have the CCJ registration cancelled. This means that the CCJ won't be registered at the Registry Trust and therefore won't show on credit searches, etc.