If a train goes via 'X', then tickets routed 'X' are clearly valid. There is surely no question about that.
It does get a bit more complicated when a train that would
normally go via 'X' is diverted off the normal route and avoids 'X'. In that case, tickets are still deemed valid but ticket booking sites will often refuse to sell the tickets!
Even worse, if the vast majority of tickets for certain flows are routed '
Not X', and the train is then diverted via X, ticket booking sites will attempt to charge you the premium for going via the longer, more expensive route! I am unsure if there is an 'official' line here, but I'd like the opinion of someone like Ferret on what would happen if someone bought the usual Leeds to Sheffield Not Doncaster tickets, and used them on a Sunday when diversions elsewhere were causing XC to be only running only an hourly service via both Leeds
and Doncaster. Is
that valid?
And don't expect to be sold the HW ticket unless you ask specifically!
I would consider that to be a potential breach of their obligations under the Retail Standards Guide.