We have had reports in the past of UPFNs being settled in instalments. I am not sure what the significance of consumer credit arrangements is.
The Consumer Credit Act (1974) requires businesses that lend money or offer goods or services on credit to be licensed by the OFT.
I'm not saying collection by instalment hasn't happened with some TOCs, but in my long experience from the fare collections & prosecution of offenders perspective, it has always been the practice to avoid setting up instalment plans for paying rail fares for this reason. Under the CC legislation other protections & procedures come into play if the person who owes the money fails to pay.
A one journey rail fare is due in full at the time of travel and not later. It isn't something that can be paid for a bit at a time. Your employer may purchase a season ticket for you and recoup it by paybill deductions, but that is a different matter.
I must stress that this is a wholly personal view, but I'm not a fan of the process of charging the full fare again for a journey when a traveller holds a ticket that is not valid for the train that they are on simply because of a booked time restriction, though I fully understand the business case justification.
If the terms & conditions relating to the ticket state that 'buy a new ticket' is the remedy for failing to comply with the T&Cs accepted by the ticket holder, then it is up to the ticket holder to ensure that they are on the right train.
Again, this is a wholly personal view, but I am of the opinion that the old-fashioned basic principle of excessing a ticket from "what the traveller has paid, up to what the traveller should have paid" for flexibility where there is no evidence of intent ought to be a better remedy, but that isn't the current situation with these tickets.
In the circumstances described by the OP, I think it very worthwhile to politely try to persuade the TOC to take a lesser, but manageable payment to clear this in one go and if they are minded to grant a slightly longer period in which to clear this so much the better.