This argument has been going on since the movie came out. There was a whole report in the Daily Mail about the 12A cert, and how the reporter thought it should have been a 15. To me that argument makes too many assumptions. It assumes that some how, watching a movie will in some way warp a child's mind and turn them into serial killers. Even in the short term, if a child is frightened by a movie, they will have forgotten about it by the time they arrive home.
Then there are those who say that the movie glorified knife crime. That couldnt be further from the truth. The message the movie was giving out is the classic "Crime Does Not Pay".
To me, this whole thing is an attempt by those pro censorship people, to start a debate on our rating system. There was an attempt a few months ago, to get a new law passed in parliament, which would have given anyone, the right to appeal against a movie rating. This, in affect, would have given right wing, pro censorship groups like "The Christian Voice", or "Media Watch", the right to appeal to the House of Commons to have a movie scene's cut, or there ratings increased. As that bill failed, it seems that the next course of action for these people is to assault each movie individually.
I am very much anti censorship, but if we are to have censorship, then i think we in the UK, with the BBFC, have struck the correct balance between "protecting" children, and giving adults the right to watch whatever they want. As soon as we have one group of adults dictating what other adults can watch, we are on a slippery slope.