I’m not sure what point you’re trying to make but GTR, who operate Southern branded trains, do have a large revenue protection team. A quick browse of the Disputes & Prosecutions section of this forum confirms that.
To clarify, you are legally required to possess a ticket for your journey if the station where you commence your journey has ticket issuing facilities, and accepts your chosen form of payment. Ticket issuing facilities are an open ticket office or ticket vending machine.
A member of staff might choose to sell you a ticket but assuming this will always happen is a risky strategy if you board a train without a ticket and then encounter a revenue protection inspector.
Don't forget the November 2025 amendment to the NRCoT which narrows in some respects, but widens in others, the exceptions to the requirement to buy before boarding. In particular, if there were no facilities to buy
the ticket you require (not just any old a ticket that covers your intended journey) or other authority to travel* because there was no ticket office open, then you don't have to buy before boarding, but must do so as soon as you are reasonably able.
If the ticket vending machine doesn't sell the ticket you require, then you might well be unable to buy the ticket you require because there is no ticket office open, and this would seem to me to fit squarely within Condition 6.1.1?
I don't know if this applies to the OP or not, but I'm not convinced the mere existence of some form of ticket issuing facilities at a station necessarily means everybody who ever boards there must always buy before boarding. Not unless the full range of tickets is available.
* the term "other authority to travel" isn't defined in the NRCoT, other than as a subset of the category "Ticket". Its inclusion here as an alternative to a ticket is, therefore, rather baffling given that the definition of "Ticket" doesn't seem to leave any room for any document or record to exist, be it physical or electronic, that entitles a passenger to make a journey yet is not a ticket. It is very difficult to know, therefore, what this could possibly mean? Perhaps it refers to Northern's promise to pay slips?