The Horrible approach, if you don't know it, is a sort of irreverent antithesis of a school textbook, and makes up one of the key trends in British children's non-fiction in recent years (it's also what I'm writing my thesis on).
Martin Barker, in his great book A Haunt of Fears (about campaigns in 1950s Britian against so-called American horror comics) mentions a bill that banned the dissemination to children of publications consisting 'mainly or wholly of stories told in pictures' portraying the commission of crimes, actions of violence and, importantly for the Horribles, 'incidents of a repulsive or horrible nature'. Punishment is four months in prison, or a fine of £100. You can see an image of it reproduced by Barker through googlebooks (it's page 16). According to Barker, it was passed in 1955, renewed in 1965 and remained in force when he was writing in 1984. I checked with a comics expert, and apparently it still applies today.
OK, I'm not serious about this, I'm only really applying it to the Horribles to point out how silly the law is. Crucially, the law notes that the publications would incite young readers to commit a crime, or enact violence or cruelty. For all the vagueness of 'cruelty' (or violence for that matter), it would be hard to apply it to the Horribles. But then, arguably it's hard to apply it to anything much. Media effects isn't exactly a precise science.
I suppose the Horribles, like the Beano or Bugs Bunny before them, get away with being so happily horrible by way of treating the whole thing as a joke. That said, as Barker emphasises, there is an intimate link between humour and the Horror comics this bill was set up in reaction to. I don't think that humour automatically excuses it, and it's not as if Bugs Bunny et al haven't been criticised for violence (think about Marge Simpson's campaign against Itchy and Scratcy). You could also argue that the law doesn't apply to the Horribles simply because they aren't 'stories', but non-fiction, facts, instead - though again, that's pretty slippery.
What this 1955 law really shows us is how much things have changed. Horrible Histories author Terry Deary (in this book, in the late 90s), argues that the whole approach wouldn't have been possible if Roald Dahl hadn't already brought about an acceptability of groteque and dark humour in British children's literature. I guess it's a truism to say people have relaxed since the 1950s, especially in respects to imagery presented to children. Plus, I think Barker's point is partly that the huge fuss made over the horror comics in the 50s, which led to the passing of that bill, was a bit of a crazy moment in British history anyway. Although with the usual suspects of Daily Mail readers, the campaign was fought by communists, worried by the influence of American propaganda.
Still, Mary Whitehouse et al have had an on-going (and surprisingly large) influence on British media (just read this book on Dr Who). Maybe there really are people who object quite strongly to the Horrible books. And maybe they have a point. I don't know, it's an interesting one.