Why does it have an 18 rating (US is rated R)
There's no way this should have this high of an age rating. At most it should be rated 15 (and whatever the equivalent is in the US with the MPAA).
Yeah it has some people getting shot and some bad language, but I've seen worse in loads of 15 rated films. It's not like the movie goes into extremely high detail with anything, you just see a bit of blood at most.
Comparing it to other films rated 18, this is mild.