Well, one's French and one's a low budget exploitation film. Whether or not Irreversible is an arthouse or film or not is debatable, but it is French, and French films always seem to be treated as arthouse films by default. The BBFC have shown elitist tendencies since forever; your average cinemagoer is stupid and easily influenced, so they're more in need of protection than the educated folks who watch arty films you see? Censorship in the UK has had this nasty stink to it at least as far back as Victorian times: the subtext that the general public are soft in the head and need to be protected from immoral influences by their betters.
There's that famous quote from the prosecutor in the "Lady Chatterley's Lover" trial in the '60s who asked the jury, "Is it a book you would wish your wife or servants to read?" Things have improved, and I can't imagine anyone publicly defending censorship by asking that sort of question now, but I'm convinced that some of that attitude persists to this day. I Spit on Your Grave is a pretty unpleasant film, and it should have an uncut UK release. It's farcical at this point that the BBFC still somehow consider it dangerous, especially as you can buy an import uncut release pretty trivially should you wish.
I have no problem with age certification (though I believe that if it's legally mandated it should also be free), but I have a very big problem with anyone telling adults what they can or can't watch or buy (outside of things that are clearly illegal and require harm to create).