Err, just checked the official site, it doesnt mention ID at all. Have you a link for that information ?
Your photo is on the ticket, that's the only id you need , surely?
from glasto site.
You do not need ID to enter the Festival. All tickets will be personalised with the passport photograph submitted by the ticketholder during registration.
Every ticket holder will be required to present their personalised ticket. Checks will be made at the gates. If you are not the person whose face is printed on the ticket you will be denied entry. "