the thing is thou - gore and violence means nothing anymore. if the alien films were out now - i am certain they would be classified as 15. so this movie may be very gory - and still be a 15.
The last I heard about this months ago was that it was going to be released as a 12A! After seeing the trailer I doubt that very much, which means smuggling my then 10 yr old son into seeing this will cause a few problems.
It's R in America, so by default it will be at least a 15 here, unless 20th Century Fox decide to edit all the violence and language out for a 12A release but a 15 or 18 on DVD. I really do think this film will be released as an 18.
The Alien films were submitted for classification though for the Quadrilogy DVD set and they were not re-classified so all 4 remain as 18 rated films.