Microsoft's outlined the UK pre-order bonuses for Halo 4 ahead of its release on November 6th.
It seems character skins are the order of the day (and some Avatar love as well). The list so far looks like this:
GAME: Downloadable Blue Circuit Armour Skin, Arctic Weapon Skin and 6 specialisations for use in Halo 4 "Infinity" Multiplayer experience.
HMV: Downloadable Locus Helmet, Purple Spartan Warrior Armour and Xbox LIVE Unicom Avatar Armour (if ordered in-store)
Tesco: Halo 4 T-shirt and Downloadable Black Raptor Armour Skin
Amazon: Downloadable Gold Web Armour Skin, Bulletproof Emblem and Spartan Emblem
Blockbuster: Downloadable Xbox LIVE Spartan Warrior Avatar
Play: Downloadable Deadeye Helmet
Shop2net: Downloadable Assassin Emblem
To have a gander at the custom armour and helmet types, head on over to the game's official product page.