Since the co op, I don't bother bringing any food or mixer. Co op has cans of sprite, coke etc. Just take a few cans of cider per day for the mornings then switch to spirits and mixer later (spirits decanted into a few re-usable water bottles beforehand). Co op also has snacks like cereal bars and fruit, then use the fantastic food on site for main meals.
I still like taking a chair but have switched to one of those ultra lightweight ones that pack up to the size of a small loaf of bread. Still pretty comfy, you are just a little lower to the ground that the more traditional camp chairs. See tickets coaches can be hit and miss whether they let you take trolleys on the bus so I would avoid it if possible.
Don't bother bringing a portable shower or wash bucket. Go to the greenpeace showers and get a token in the morning and come back in an hour or two and your number should have been up already. Or go around 5pm and there is hardly ever a queue and you can go straight in.