The only time I heard of an Advance ticket holder being refused the chance to pass a barrier was Clapham Junction.
I've been refused access to the 'outside world' at Hereford travelling on advance ticket, despite serious TfW disruption that ultimately ended up with all passengers travelling on TfW to Newport, Cardiff etc. being delayed for five hours. The insistence was that my ticket did not allow me to leave the station here and that I should remain on the platform or train and wait for announcements.
That being said, I was offered an apology on noting this after my arrival at Newport later that evening, and they had no issues letting me BoJ despite being in the middle of organising taxis for myself and others.
Barrier staff will be 'rogue,' as it were. Does anyone know of any rights that the advance ticket holder would have in those situation? Other than perhaps some law that deals with being kidnapped/unlawfully detained!