I was led to believe that you cannot purchase a priv ticket in advance but only on the day of travel.
RST where you can travel pdf doesn't appear to offer much clarity either.
How to buy a Priv-rate ticket, and what to do if this is not possible
At present it is not possible to buy Priv-rate tickets online, by phone, or
from ticket machines.
Where the station has a manned ticket office, you must purchase a valid
ticket before you start a journey, irrespective of the length of queue or
wait time.
If the ticket office is closed and local ‘promise to pay’ tickets are available
from vending machines you must obtain one before travelling.
If you board a train from a manned station without a ticket, or from a
station where ‘promise to pay’ tickets are in use and you haven’t obtained
a ‘promise to pay’, the train operator is within their right to refuse to sell
you a Priv-rate ticket.
Only if the ticket office is closed, and there is no local ‘promise to pay’ in
use, can you buy a Priv-rate ticket on board a train. You must actively seek
staff on-board the train, have your fare ready and offer to pay it at the
first available opportunity. By offering to pay there can be no question
that you are trying to travel without paying.
If you don’t actively offer to pay, then you may be accused of fare
avoidance, and risk committing a Travel Irregularity. (See the separate
section on Travel Irregularities for the consequences).