I recently made a booking for a future journey from London to Fort William and found the process quite confusing. The nice man on the phone was quite happy to take a PRIV booking for me, but matters were complicated by travelling with Her Ladyship who, due to circumstances, is not entitled to PRIV-rate travel (of which more anon). The only restriction I was told about was not being able to book a double or club room, but only a seat or a standard berth. Quite how he would have gone about verifying my eligibility for PRIV-rate travel was never explored as we never got that far down the process.
The booking was complicated by the fact that travelling on a PRIV meant that I would have ended up having to make two separate bookings, one for me and one for her, which would have seen us both in different parts of the trains. I had imagined that it would be a matter of buying tickets at whatever rate was applicable and then securing our accommodation, but this appears not to have been the case. In the end I had no alternative but to book at full rate so that we will be able to travel together and have the en suite facilities that she would consider to be a basic requirement.