Alien would probably get a pg 13 if the chest burster scene was edited differently with alot less blood, it would get an R if it was as bloody as the 1979 theatrical release. Take most of the f bombs out've aliens and make the colonist chest
burster scene less gory it would get a pg 13 too. It wouldn't really matter in the uk as they would probably both get 15's if released today, the uks 15 rating is quite broad as long as it dosen't dwell on the gory scenes too much. Cloverfield is a 15 in the uk and a pg 13 in the U.S. The Dark knight was borderline 12a, Prometheus looks very dark and intense i think it will receive a 15 just for that purpose.