In Britain the 18 rating is generally box office cyanide, barring as it does the main cinema market of teens, although some horrors and the occasional drama (Fifty Shades of Grey) manage to succeed with an 18.
Warp have frequently been hit with an 18, This is England being a notorious but typical example - intended as a 15, the BBFC decided it's strong language (scripted by the teen cast themselves to a degree) and one violent scene made it an 18. Many newspapers noted their disagreement with this judgement, and several local councils used their discretionary power to make it a 15 in their area. See also Ken Loach's social realist Indie Sweet Sixteen (18) and the studio tentpole Dark Knight (12), or WT's World's End (15) for could be seen as a system that favours the studio releases.
The documentary This Film Has Not Been Rated exposed a very clear bias of the US film censor towards the studios, with South Park's Trey Parker noting the contrast between the help he got to keep the South Park movie rating down and the stony silence he got when producing his debut film, a low budget Indie.
Rob Zombie has just tweeted his relief at getting his next film, 31, down to an R rating from the NC-17 it had twice gotten from the MPAA - whilst promising fans he'd release the uncut NC-17 version on DVD. He also released the poster (which takes the same basic design approach as Warp's '71 I noticed!) on his Twitter feed, a nice example of convergence and the potential importance of social media to Indie filmmakers (do Warp use it as well as they could?).