I thought it was permitted too. I thought "for interchange purposes" only precluded, for example, buying a ticket from Basingstoke to Reading West and travelling to Reading General before doubling back to Reading West. In this case that would be totally meaningless anyway since the fare would be issued to Reading Stations, but would make more sense somewhere else where a routeing guide group didn't coincide with a fares group. For example, Stansted Airport. It is in a routeing guide group with Stansted Mountfitchet but they are not a fares group. It is not permitted to buy a ticket from London Terminals to Stansted Mountfitchet and travel to Stansted Airport and break your journey there, saying you are going to travel back to Stansted Mountfitchet later - because the destination on the ticket is within the routeing group. But if both origin and destination are outside the routeing group, then doubling back is allowed, I thought (e.g. London Terminals to Elsenham) - and none of this has any effect on break of journey restrictions since they apply to the ticket and the routeing guide has no effect on them.
But I did read or hear somewhere I think that ATOC had recently or were going to "clarify" this and change the rules so that break of journey would not be permitted whilst doubling back for interchange purposes. I can't see how that could be codified into ticket restrictions though.
