I don't think that's right, as I can't see a relevant easement (there are easements involving Romsey, including one that makes no sense to me, but none appear to apply here, and there isn't a group that would allow doubling-back and even if there was that doesn't apply if the origin and/or destination are in the group).
However if you book it online, print out the itinerary, and carry it with you, it could be argued that creates a binding contract. Having a seat reservation and sitting in the exact seat possibly further strengthens your case. I would strongly advise against buying a ticket by any other means and doing such a journey. It is additionally worth printing out an itinerary on the National Rail website (if it will provide the same itinerary).