Apologies, I misunderstood your question. It might be broken, as it was in the case recently where I tried to make a reservation but couldn't - I found out why when it arrived, it was a booked HST worked by a Voyager. The different seat numbers and lack of coach G meant that all of the reservations were cancelled, so they didn't take any TMRs. It could be possible that they know the reservation system on a Voyager has failed, and have accordingly ditched it but it's far more likely that there aren't any seats left!
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You do if it is too full to physically board - in my view you should be accomodated on the fastest available alternative service(s) to your destination (regardless of operator etc.) where that is the case. However I do take your point, and I have been unable to board a train for which I had an advance for in the past due to it being half-formed and was not afraid to just travel on the next train, but recognise many people would be somewhat scared of that. I don't think there's actually protection for this eventuality in the passenger's charters or Conditions of Carriage either!
As far as we are concerned you're stuck with the next train operated by the same train operator with advances if your train is too full to board, unless they bother to ask for ticket acceptance, which is generally granted. On a personal level discretion is more likely to be exercised though.