It was part of an ongoing police investigation into black people carrying firearms. Given that such people are want to shoot said firearms, it's not unreasonable for the police to also be duly armed. Likely it was a special unit that is trained and given clearance to shoot black people should they be found in possession of firearms.
It's because of this that the citizens of the UK can sleep safely at night, and need only kill a black person should one be found burglarising their own home and castle.