They will NOT have a specific seat reserved, but WILL be booked train only.
That, at least, is something. The horrid experience on CrossCountry where unreserved passengers can be turfed out of their seat mid-journey, or the "ggf freigeben" equivalent on
DB» , adds needless stress to a journey.
That said,
GWR▸ offering same-day advances just reinforces my impression that they are just not very good at doing yield management through walk-ups, as the Cotswold fare changes demonstrated.