[snip] ... Note that if you travel outbound on a via Taunton ticket, you need to come back the same way - you can't drop back to the cheaper route via Trowbridge.
The Man in Seat 61 gives an example that appears to cover this with a journey between Reading and Exeter(?) for which there are only two routes given - via Taunton or via Honiton (there is no 'via any permitted route') - one is more expensive than the other - can't remember which. Having bought the most expensive route return, when travelling back via the cheaper route and not via that place on the ticket, he suggests that the passenger asks for an excess fare ticket to be issued, and the person issuing it would find that its price is zero.
That should not be necessary. Where there are alternative fares for the same journey with different "via" routeings, a more expensive fare is always valid by any cheaper routes. As the instructions document to the Routeing Guide puts it:
Where the fare specifies a particular route, there will be restrictions in the fare routes data that will affect thee routes listed in the Guide which pass through the station shown in the route description... Where there are other permitted routes, these may also be used for the same journey provided the same or a lower fare applies
In some cases the tickets make this clear. The most expensive ticket is likely to be marked "valid via any permitted route". This is the case for the
GWR▸ -priced ticket from Wokingham to London that is valid via Reading. The cheapest ticket for this journey is marked "valid only via Staines", as it can't be used on any other route.
There is another fare in between those two, via Guildford. This route's short name is "Ascot/Guildford", which is confusing. As I've never bought one I don't know how the tickets are marked. However, the
TVM▸ information for the Ascot/Guildford option on the selector page says: "valid only for travel via (changing trains or passing through) Ascot or Guildford". Given the ticket has space for just oner line of routeing text, I imagine that would be "valid only via Ascot or Guildford".
As far as I know it is permitted to travel on a train with a ticket for the right journey by the wrong route. You just have to pay the relevant excess fare if asked to to so. However, the NCoT (and NCoC before that) doesn't really address the point. The bit about excess fares in the 2013 NCoC related to travel not by a permitted route at all, and in the current NCoC even that's not present. What is there mixes up a ticket being valid for your journey with being valid on the service used - so explicitly missing exactly the point in question!
In such cases, if your Ticket is not valid for the journey you intend to take, you must, before you commence your travel, either purchase a new Ticket or, (where the terms and conditions of the Ticket you hold allow), exchange or excess it for a ticket that is valid for the service you wish to use