Showers at festivals are non-existent or require long queues for short cold showers. Take plenty of wet wipes and a roll on deodorant.

Socks, more than you think, whether it be wet days or cold nights you will usually find you get through more socks than you thought.

Sun Cream... Even if you don't usually burn being outside all day and likely drinking if it happens to be hot can fuck you up, and whilst sunburn is manageable the associated dehydration can ruin the next day.

Bin bags. Both for tidying up your own litter around your tent and for keeping wet clothes, worn clothes and clean clothes separate.

Two torches. Small one to keep on you at night as some camping areas are pretty dark and drunkenly navigating a maze of tent ropes in pitch black is no picnic (Some are well lit, never done Shambala so ca't say whether it specifically applies but for something the size of a torch I'd always go better safe than sorry) and a second one for a spare to leave in tent so you can still find your stuff if you lose your carry around torch.

Paracetamol. Alcohol, drugs, very little sleep, loud noises, long days in the sun are all a recipe for headaches from hell. Drink plenty of water and keep pain killers on hand / in your tent.

Something warm, even though it's August and even if the forecast is good, it can still get very cold overnight. A hoody tied round your waist all evening is less annoying than shivering when you're still out in the early hours of the morning.

Bottle of water. You can buy them in there but having one on you for the queue is a priceless blessing if the queue is slow, and having an empty bottle you can refill to leave in tent for over night is always handy.

Apart from a change of clothes and obvious things for any night away like toothpaste / toothbrush / etc. Think that is everything I'd consider as essentials.