This is what I believe to be Syd's Border Police / Immigration Cop uniform.
Since there's no tags at all on this costume, I had to do some homework before sharing it here.
The first clue that leaded me to think it was related to the psycho cop brilliantly portrayed by Peter Mullan is blatant : Unlike all the other Immigration Cops seen in Children of Men, Syd's chest armor doesn't have any ammo pouches on the back. That's also the case with the chest armor shown here. Even the only other officer seen on screen, the Border Police cop wearing a beret that boards the Bexhill bus, wears ammo pouches on the back of his chest armor…
Little details spottable here and there match scratches on the chest armor seen on screen. I can't tell if the other "soft parts" composing this costume were also originally part of Syd's uniform (it would be logical though) but I'm 100% sure the chest armor is the one seen in all the scenes involving Peter Mullan dressed as an Immigration Cop.
The costume consists of :
Black beret
Wireless headset
Black jacket
Black trousers
Black gloves
Black chest armor (including front ammo pouches filled with rubber ammo magazines)
Utility belt (with tri-fold restraints, baton holder, telescopic baton, CS gaz holder, drop leg holster and ammo pouches)
The CS gaz canister is missing.
A pair of black combat boots 100% similar to the ones used on screen and an airsoft H&K USP pistol similar to the one used in certain scenes by Peter Mullan have been added to complete the display.
The costume is in very good condition and is very impressive to look at in person. Very sinister overall look that reminds of SWAT and even Nazi uniforms, I guess that was the idea... Its screen used parts came from two different sources, one directly attached to the production of the film.
NOTE : I also have a helmet (with goggles) and an airsoft Heckler & Koch XM8 rifle (with dummy sight casted from a Heckler & Koch G36K). Both items were screen used and can turn Syd's Boarder Police uniform into a regular Immigration Cop, like for example the ones seen at the train station when Theo meets Jasper in early scenes of the film.