Off to watch Saturday, surprised that's it's only a 15 over here considering it got a R rating in America.
R means anyone under 17 can attend with an adult, so not really the US equivalent of an 18 cert. R will almost certainly get a 15 cert over here.
NC17 is strictly 18 and over. Such films would be more likely to get a 18 cert in the UK.
Maybe the classification board in the US are more strict, so I wouldn't be surprised to see a NC17 film get a 15 cert from the UK classification board. I'd be more surprised to see an R film get an 18 in the UK.