I did Bestival last year for the first time and whilst you're not allowed to take your own booze into the arena they were very lax with the checks.
They asked me if I had any drinks in my backpack I said no (I did), they let me in. Someone was carrying a water bottle of vodka, they asked "what's that?", she said "water" and they let her in. Even saw someone with two cans in their hands pull their sleeves down to cover them and they were fine! All depends on the security on the day I suppose.
Also didn't get checked on entry for quantities as I had spirits (in a plastic bottle, of course!) on me. But again, I expect if you get pulled for a search they'll confiscate.