I had to buy a 2 day ticket yesterday. With uni and stuff I didn't know till now if I could go, and I actually don't know whether I can go both days, but all the single day tickets had sold out. I will definitely be there on the Saturday but I'm not sure if I can afford the train journey to Leicester both days yet. All train tickets out of London are stupidly expensive.
All the years before this I have bought on the day because, generally, it's been really late before I knew if I could go. I dunno if it's a good thing having all the stock available online. I think it means folk who have more complicated schedules are more likely to miss out.
The website is a bit misleading, too. It says that tickets will be available on the door still - or did say so yesterday when I bought mine. I suspect some people will still think that is the case - you might want to do something with that.