While you won't find any mention of it in the current rating page, the eagle-eyed spies over at Eurogamer noticed an eye-opening description of "references to suicide and an implied child rape" in a summary of Metal Gear Solid 4 briefly posted on the BBFC site (this summary was likely the Extended Classification Information, which is not supposed to be posted more than 10 days before a game is released). Before you get excited, note that Kojima Productions' Ryan Payton told Kotaku that "there is no mention of rape in the game" and that the BBFC's confusion may have come from a mention of "child trauma."
Regardless of the specifics behind the disturbing description, the BBFC still granted the game a relatively tame 15 rating for violence that "may be strong but may not dwell on the infliction of pain or injury during gameplay." The fact that it's possible to "progress stealthily through the game, avoiding violent confrontations where possible" seemed to weigh heavily on the decision. Hey, you can do that in the 18-rated Grand Theft Auto games too! Well, OK, you can't really advance that way, but you could avoid violence by driving a cab all day.