There are always exceptions, and Fifty Shades of Grey was one where anything lower would have been distinctly off-putting, given audience expectations of realistic sex scenes:
Whatever happens, Fifty Shades looks absolutely certain to overtake The Wolf of Wall Street (£22.7m lifetime) to become the biggest ever 18-certificate title in this market. This is a movie where the 18 certificate can be considered in no way a hindrance – in fact, audiences would have been rightly suspicious of a Fifty Shades film that won a 15 rating. Usually, film distributors push for the lowest possible rating, but it’s easy to envision Universal asking the question of the UK censor: what exactly do we need to include to secure an 18?There are very few 18-rated Working Title films; contrarily, there are very few PG or 12 Warp films. Working Title generally aim for a mass, mainstream market, and thus mostly avoid 18-ratings. They sacrifice realism to do so, something Warp are less willing to do. If WT present an idealised representation of the UK, Warp tend to present a grittier, rougher representation, with the very title of Meadows' This is England arguably a direct riposte to the Notting Hill, Bridget Jones (etc) depiction of a twee middle-class Britain, as viewed by Richard Curtis (and savagely satirised in the Curtisland animation).
(Charles Gant, Feb 2015)