The rating will 100% be (15) in the UK because it is (R) in the States.
The only movies in the UK today that get rated (18) from a US (R) rating are movies that have ALOT of graphic violence, sexual content and bad language in them, The Amityville Horror REMAKE does NOT, it's much more of a psychological thriller then it is a full-blown horror movie.
The ORIGINAL still carries a (18) certificate in the UK today but this is only because it was rated before the BBFC changed their certificate agreements, it will most likely be re-rated in the near future carrying a (15) certificate just like a large number of other previously rated (18) movies that are now carrying a (15) certificate.
If the ORIGINAL had been released today it would have been given a (15) rating.