Can a ticket inspector or similar hold you against your will - for example, calling the transport police, and can the TP hold you against your will? I would have a tendency to give them my name and address, say I'm right, and if they're not satisfied ask them to speak to me next in court where they would have to prove I'm wrong. And walk out.
It is unlikely that station staff will be allowed to effectively perform a citizen's arrest; even if they have had the appropriate training to do so, they should only be doing this when there are reasonable grounds to do so.
A customer showing a ticket they believe is valid and is only in dispute because the staff refuse to accept the route being a permitted route would be contentious to say the least.
Another reason I would recommend having an itinerary is that it would be contrary to consumer and contract law for the company to charge you an additional fare if you showed such an itinerary. The worst they can do is withdraw the ticketand issue a free of charge replacement ticket valid to the destination and complete a ticket irregularity report (TIR) for investigation.
If they are claiming a retailer incorrectly issued a ticket, that could be seen as slander against a retailer and having the backing of a good retailer may be useful to a customer in this situation.
Ask questions rather than make too many bold statements they won't like. Don't swear or use derogatory language such as "jobsworth" ( technically they are
not doing that if they are acting incorrectly anyway!). Make notes as soon as the encounter is over so you don't forget key points. Keep calm. And if they threaten to call the police, don't then act differently, such as in a way that makes you seem like you think the police would deem you to be wrong.
If you have a witness that can be useful too.
Also an independent person e.g. a passenger you are travelling with who is on a different ticket , could be of use. I will be much more bold with what I say when I'm not the victim, as it's harder for them to do something about someone who has a ticket they are happy with; I did this on a Fife Circle train and got an aggressive Guard to stop charging other forum members who had valid tickets ( he was happy with my Glenrothes ticket).
Some rail staff can become aggresive and abusive but you must ensure you remain in the right. Do not react badly. BTP may believe staff over passengers and you can end up in more trouble if you react. If they swear at you, don't swear back. If you do, they will win and you will lose.