I used to have to travel with Virgin to get from Southampton to Birmingham and back, when I used to pop home for the weekend.
In 2001, I could buy a return for £16 if I bought it a week in advance and travelled on the specific trains at the specific time in the designated seat. If I bought it two weeks in advance, it was £13. This was all with a Student Rail card.
In 2003 and onwards, I could not obtain these advance travel tickets despite my efforts. In one situation, I went one day and asked to be told they weren't released yet and the next day that they'd all gone. Pathetic - it was easy to see they'd limited the numbers to generate more income. I had to buy a "normal" return which was £36 with the railcard. Not much now but at the time that was a month's supply of shopping.