What is the problem with booking online before you turn up? I have been doing it for 6 years and never had ANY problems.
I mean yeah, I'd love to turn up at Euston on a Friday night and queue for a ticket but I could really think of better things to do with my time.
I really can't think of any reason to turn up on the day and getting your ticket if you know you are going before had, why not just book it in advance, turn up and get on?
And besides, if you know you are going far enough in advance why miss out on cheap single fares? I got to Leicester from London for £5 couple of weeks ago, you can't do that on the day! (unless you don't pay!)
And you can still use your YP railcard to book online anyway. And if you've never been able to get a seat by not booking, attempt to go anywhere on the mainline on a Sunday night, it isn't going to happen!
I use
http://www.thetrainline.com/ for my bookings, they have a full list of what the ticket meanings are on there when you click on them