Generally speaking if it's just black you are after then Halfords Matt Black will be fine - they also do a range of camo paints in the larger branches that comes in matt black, but I don't think it's too different to the car matt black!
If you want specific "gun" paints your main option in the UK is Krylon which you can get from Patrol Base online. Or you can look for Duracoat, but that's nearly impossible to source in the UK and needs extra equipment (airbrushes etc..) to do properly.
Although when you say "touch up", depending on what it is you're trying to fix I think you'll find a bit of wear and tear actually looks better/more realistic than a perfect finish!