Yes this is true, only you have to be much less flexible and book yourself on to a specific train (date and time) and if you go on a different train to this your ticket is invalid. Just go into your train station and they'll be able to book you an advance ticket from there, and tell you all the different savings you can make ( it might vary from area to area but with mine, its cheaper the longer ahead you book it, so if you book a month in advance its really cheap, 2 weeks in advance and its more expensive, one week in advance, bit more expensive etc) Otherwise, virgin trains website do sicounted tickets, and the trainline.com.