That's my point - I can see a train company arguing that you can still obtain the "ticket you require" before boarding, you just can't do that by excessing your existing ticket.
I'm not saying that argument is necessarily correct, but I can see a TOC trying it. In any case, I do think the current wording is too ambiguous.
I agree with that. Ultimately the existence of the right to buy the ticket you want and to get a change of route excess if you need one, may depend on people being willing to insist on that right, and regrettably
that may involve facing down crude threats of penalty fares, prosecution and/or civil action from TOCs. It's for individuals to weigh the risks involved.
I did a journey late at night involving a tight connection at Aylesbury a few days before the NRCoT changed. If I missed the connection I would have missed the last train for which my ticket was valid and would have needed a change of route excess to get home, but it would have been impossible to buy one before boarding (or at all, probably). With the NRCoT as it was at the time, I was able to take the risk.
With the NRCoT as it is now, I would have to choose between
1. Buying an Any Permitted return ticket at twice the price of the via Chalfont & Latimer (i.e. overpaying)
2. Trying to get a one-way change of route excess for the return leg at the time of buying the ticket, or afterwards but in advance of travel (i.e. overpaying since in the event I did make the tight connection and did travel via Chalfont & Latimer),
3. Facing the risk of a penalty fare or prosecution on the return leg if I missed a tight bus-to-train connection and was then revenue inspected travelling in breach of a route restriction on my ticket, or
4. Driving.
I would personally take the third option and rely on the unavailability of facilities to buy a change of route excess at Aylesbury at the time I got there, but I can see why many people would take one of the other options.
If this were about revenue protection, by the way, the facility to excess a ticket online would be made available. Or, at the very least, a permit-to-travel-type facility would be made available specifically for excess fares, so if you know how much the excess costs, you can choose to pay that amount (either online or at a TVM) in return for a voucher which you can then use to buy the excess on board or at your destination. That way, if nobody is around to sell the excess, the TOC still collects the revenue. The fact nothing like this is being done is probably down to laziness, although it could also be taken to indicate that the railway wishes to be able to overcharge people.