I think AvP wa a 15 in the UK due to the suspense and appearence of the characters (they deemed it too frightening for people of 12 years and have learnt from the 12a rating of T3). But unlike films such as Hostel or Saw (both 18), the violence isn't being inflicted by another human (and thus warping teenagers fragile minds, like they believe Manhunt did etc and creating copycats) but by a monster.
Not many monster movies garner an 18 rating here unless there is some kind of human involvement in the deaths.
