The OP seemed to suggest that they wanted to take an earlier train that would’ve got them to their destination early. That’s starting to smack of using an advance ticket as if it were an open ticket.
The TOC isn't providing the service that was sold to the passenger as part of their itinerary. If needing to be there by a specific time, why
isn't that reasonable? In my book it absolutely, 100% is.
Flexible tickets are provided for people who wish to pay more to be able to choose when they travel
regardless of delays.
If the TOC, for whatever reason, isn't honouring the original itinerary, then it's a lot friendlier to allow the passenger to choose an alternative that meets their needs than to
cost yourself money to not permit them to do so.
Personally l can find no way that the contract text could be interpreted as permitting use of a train scheduled to depart prior to the planned service.
Not even if that train is late, and ends up departing after the planned one's scheduled departure time, i.e. it is the next train to depart to the ticketed destination? I'm surprised you can't see that interpretation.