
Hi,
Have you tried phoning the ticket office?
Maybe they could take your order over the phone & you collect your tickets when you arrive? I was able to do this when I decided to stay for BIS last year but as I was traveling on the ferry the next day it would be silly for them to be posted so I collected them at the ticket office when I got there.
ETA - 0044 0844 444 9944 this is the ticket hotline.