Most places in Basildon are fine and those on the avoid list I would venture into with no issue, except save The Barge (but that ony becauseI haven't been in there for about 20 years! You may get a frosty reception in a large p*ssed up group but that's usual if you're not considerate and happens anywhere.
The town centre area is no problem during the day with many shops to browse. By night unfortunately Basildon has no venue to venture onto after the pubs so it tends to attract the people who can't get into the festival park.
The best places to hit are now on the festival park, save Sugar Hut in Brentwood.
There is a smart casual dress code. No sports gear. No trainers, sandals, flip flops etc. Some shoe type trainers are accepted BUT best bet is proper shoes.
I understand that the unofficial party will indeed be at Chicago's
You will need Millennium ID. Those without Millennium ID may still get into Chicagos before a certain time which I can't remember what it is (9pm rings a bell).
General public will also get access.