Ok the customer has a valid contract (due to having been executed in compliance with the law) for conveyance from origin to destination (at as close as practicable to the originally booked times) even if the train company changes aspects of the service such as removing an intermediate calling point.
Other examples of where a contract remains a valid contract include where the train is retimed, cancelled (you can take a train before , or after, subject to any operator restrictions as applicable) or diverted.
In the case of splitting at WOP, the purchase of new tickets as described wouldn't be allowed once the train is no longer advertised to call there, but the concept of a valid contract remains in place for pre purchased tickets which were valid tickets at the time of purchase.
The potential confusion here is that a valid contract under contract law is not quite the same concept as determining validity of a ticket (or combination of tickets), at the time of use, if the terms and timetable advertised at the time of use are considered without knowledge of the terms of the contract.
In other words, a system which determines validity of a fare may return an 'invalid' response but that doesn't invalidate the contract in any way.
Other examples where the two concepts can be at odds with each other are where a ticket is used for a route that deemed (rightly or wrongly) to be a valid route by a booking engine at the time of sale, but not deemed valid by another party at the time of use.
The National Rail Enquiries site does state that tickets purchased online are valid by the route shown on the itinerary; this wording was added to clarify that contract law does apply (which is staying the obvious) though I would like to see the wording changed to ensure there is no room for misinterpretation by people who are less familiar with contract law. Furthermore I would like this to be enshrined in the NRCoT, though the omission of such clarification in no way alters the legal position.
If a ticket inspector believes tickets are invalid there are procedures to handle this, which involve investigating the cause of the issue, but do not involve charging the passenger any additional fare, in compliance with the law.
