If you don't get in on the presale, I wouldn't worry too much. Last year we got tix for Saturday at the box office around 11am that day, and we only bought our Sunday tickets with Tool and Madonna headlining about 3 weeks ahead of time.
i can't believe ticketmaster thrift you of so much 'convenience charge'. its like 4 pounds for shipping and service for glastonbury and thats a super expensive festival too. they also have nice cloth wristbands that you definitely can't slip off for somewhere else