I'd rather have the BBFC than the MPAA, but i do think that they're both becoming fairly obsolete thanks to the internet. As has been mentioned, anybody underage can acquire a film that the BBFC deem to be unsuitable for them. Additionally, you can now guarantee that any film that the BBFC bans will be online even before they've banned it, for example: a disturbing horror film i watched recently called 'A Serbian Film' is highly likely to be banned or at least edited beyond recognition by the BBFC, and yet thanks to the internet anyone can watch it.
Then we get into the argument of who says that a responsible adult should have their viewing vetted by a ratings organisation? I'm all for protecting people under 18 from seeing films like 'A Serbian Film' (a strictly for adults film if ever there was one), but why can't a responsible, mentally sound adult watch it? In my opinion, the BBFC should only be allowed to provide in depth information on why a film has been given a particular rating, they shouldn't be allowed to ban things (unless a "film" features real killings or something, in which case it's understandable).
As for the MPAA, isn't it utterly pointless? Companies are allowed to release films on DVD as "Unrated", effectively bypassing any age restrictions.