Nobody keeps statistics, so it can only be anecdotal of course.
But given Jewish people make up a tiny proportion of the British population (0.3% I think), then out of 125000 paying attendees, you'd expect only about 375 of them to be Jewish. I personally know of at least 50 who went this year (personal friends, festival friends, and kids of friends), and randomly found myself amongst a crowd of over 20 at the Pyramid whilst waiting for Elton John.
I'm no statistician, but knowing what I know about secular Jewish culture, and given I only ever personally know or meet a tiny proportion of all the people at Glastonbury, I think that by extrapolation it's reasonable to conclude that Jewish people are over-represented at Glastonbury compared with their numbers in the general UK population.