Whilst I would never condone or recommend deliberately breaking the T & Cs of a ticket, I cannot help but note that, if a Liverpool- Tamworth Super Off-Peak Return [which has restriction code LK preventing BoJ on both legs] is purchased from a trainline-based website, and link on the link to the "ticket restrictions" for the ticket purchased, as described by the website, it states:
"BREAK OF JOURNEY
You may start, break and resume, or end your journey at any intermediate station along the route of travel."
If I didn't know better, I would naturally assume that BoJ was permitted, and indeed potentially enter into the contract for travel on the specific basis that I wished to break my journey, and that BoJ was permitted.
The NRCOC permits BoJ restrictions provided they are "made clear".
The precise wording from NRCOC 16 is:
However, these rights may not apply to some types of tickets for which a break of journey is prohibited, in which case the relevant Train Companies will make this clear in their notices and other publications.
I'm not sure this could reasonably be said to apply to the well-hidden and opaque restrictions on BoJ on "flexible" tickets, especially so as some tickets even have different BoJ restrictions depending on the actual route travelled on for that ticket.
For example, I am told by East Coast that a London-Glasgow Off-Peak Return
permits BoJ when routed via Kings Cross or Stevenage. This is as I expected, and presumably because this ticket takes restriction code 5F (forbidding BoJ on the outward leg) if routed other than via Kings Cross or St Pancras. If routed via St Pancras, it takes restriction code 9J (permitting BoJ on both legs) and if routed via Kings Cross, it takes restriction code 9F (again permitting BoJ).
In response to Yorkie's claim that all Off-Peak Day Returns permit BoJ; this is no longer true since "Simplification"; a Rugby- London 'London Midland only' Off-Peak Day Return has restriction code SK, which prohibits BoJ on both legs, (although, once again, the conditions listed trainline-based websites will tell the purchaser that BoJ is always permitted on Off-Peak Day Returns)
This is ludicrously complicated and difficult to enforce in practice, especially if a passenger claims to be merely "stopping off to use station facilities", which does not count as a BoJ.