According to the
Glasto 2009 map it looks like West is for family campervans, however if they are now asking you to stipulate in advance then they ought to have pointed that out as you won't be the only person. Perhaps there will be changes to the west and east family campervan areas.
Also if anyone else is yet to book campervan ticket, west was a nightmare to get out of last year and all the smug campers going out of east car parks drove straight out. I'm sure this changes year to year.