Assuming the passenger is on board with good reason not to have bought a ticket before boarding, that surely offends the conditions of travel:
14.1 Some Tickets specifically exclude their use in conjunction with other Tickets. This will be made clear in the terms and conditions when buying such Tickets
14.2 Unless Condition 14.1 applies, you may use a combination of two or more Tickets to make a journey provided that the train services you use Call at the station(s) where you change from one Ticket to another
Not forgetting The Summary:
The key responsibilities of Train Companies (‘we’) are:
• Prior to departure and during your journey, we will make available information that will help you to plan and successfully complete your journey on the National Rail Network. This will wherever possible be done in a way in which people with sight and/or hearing difficulties can understand.
• We will make available clear information about the range of Tickets to help you make a well-informed choice about the most appropriate and best value Ticket for your journey.
What if the passenger knows what they want? I gave someone a note to give to the guard on a Cross-Country train to split their Oakham-Exeter ticket into 5 separate tickets - no problem. That was 4 years ago, mind.