There is plenty of time yet for them to plead guilty. Especially when they see you are willing to attend. They can do this even after you have arrived at court.
Exactly. A defendant can change their plea from not guilty to guilty at any point up to the start of the court hearing (or even beyond).
It's fairly common in the Crown Court for a defendant to maintain a not guilty plea in the hope that (as sometimes happens) a crucial prosecution witness doesn't show up. When you do arrive, be prepared for the possibility that a change of plea would mean that you don't need to give evidence after all.
And definitely get in touch with the Witness Service before the trial date. They will look after you while you wait to go into the courtroom, usually (depending on the court facilities) in a private waiting area to which the public doesn't have access. You can also ask them to arrange a pre-trial visit to the court if you think that will help.