If the OP failed to pay, as you advised, then yes it could possibly go to county court, as it is a civil debt and money has to be reclaimed. Credit scores can be adversely affected by a number of things, including applying for credit!It cannot affect your credit score until such point as they take you to County Court, obtain a judgment in their favour, and you fail to pay the judgment within 30 days.
That might be how most Tocs work (although Regulation 12 of the Regulations for Mainline railways have a specific section allowing for this to take place, http://www.legislation.gov.uk/uksi/2018/366/regulation/12/made, London Underground issues Penalty fares under separate legislation). However this Penalty fare was not issued by a Toc, it was issued by London Underground who contract Ital/Ircas to manage its Penalty fares (post issue) and yes, they have hundreds, possibly thousands, of Penalty fares taken through to the recovery stage. Previously, when they handled this in house, they were cancelled if not paid, however this has not been the case since the start of the decade.In any case, have you any reports of Penalty Fares being taken to County Court? It is totally unheard of; in virtually all cases, unpaid Penalty Fares are cancelled and prosecution considered.
Says who? It is a civil debt and charges are added in accordance with the debt collection agency's own rules and policies, just as if you (say) failed to pay your credit card bill.Any alleged additional charges beyond the undiscounted Penalty Fare must be based in the relevant Regulations. There is no facility in any such Regulations, as far as I am aware, for Penalty Fares to have additional charges added on for debt collection.