How about a boho midi dress with boots? And whatever fun accessories you would like to add?
Anthropologie have a sale still going on, Boden have a sale with midi and maxi dresses, Top Shop might have something funky. ASOS have some midi dresses eg this or this
Wellies, esp pretty or fun ones, would play to the Festival theme, even if you choose a conventionally pretty dress. They would be very useful if it is raining, especially if the venue is muddy or you are spending a lot of time outside. ASOS here might be a bit groovier than Joules I guess.
You could take some shoes suitable for dancing if this is going to happen at some point in a dry part of the venue. Perhaps in a small rucksack - very festival. It is handy to have something containing a water-bottle or small snack, in case of delays (very festival). A torch and some loo paper would be authentic
. Also perhaps some sort of packable raingear or brolly.
I think of maxis as a bit more festival gear than midis, but again midis would be practical if it is going to be muddy.
Are you taking a hat of some sort?
(Am vicariously enjoying looking. I have been to a number of festivals this summer. But we have given one a miss this weekend, which might be good given the downpour of biblical proportions we have had. It all depends a lot though on whether your friends are thinking of a 60s or 70s inspired festival, or Summer of Love, or are hipsters? The most festival-like wedding we went to was fabulous fun, hope everyone has a good time).