Pack food, warm kit, a water bottle and a wide-necked bottle for urine (in case you get kettled). I'd recommend chocolate bars as they can be stuffed in a daysack and forgotten about until needed. Strip your wallet of anything you don't need and leave it at home. For example, a driving licence; they can be used to cross-reference you, and a minor discrepancy such as a recent house move not updated on your licence can be used against you.
Transport For London (TFL) internet search to satisfy transport, lodging queries. Mobile phones don't work on the Tube (unless further afield where the Tube breaks out from underground.)
Mobile phone with video capability - remember Ian Tomlinson and the way his killer was trapped.
A small first aid kit - plasters and medical scissors to create butterly stitches. Welcome to the world of the head wound; other favourites include baton strikes against muscular parts of the body such as the outer thigh or shoulders. A strike there paralyses for several minutes. :)
Get into the habit of filming your mates as tempers rise; it could be invaluable evidence later.