The reason why there's so much confusion and for so long (and why we had such stupidity as the Mull of Kintyre test) is because of the sheer lack of cases brought under the Obscene Publications Acts and because of the woolliness of the statute. Furthermore, working out what, legally, is likely to be held as obscene is difficult because when there are cases, they have also centred on who is likely to access the material.
As a result of this, this not only leads to official censors (BBFC, IWF, etc.) taking a rather broad approach to it, but also a culture of self-censorship amongst writers and artists and similar. This is quite unlike the concepts at English law of indecent images of children where there is a lot more case law and where it's a lot better defined, and where authorities have a scale (the "COPINE" scale) which sets out how potentially indecent images of children are classified themselves, mainly due to the fact that child pornography is treated colossally seriously by the powers that be, given that it is in itself images of actual child abuse, whereas other stuff... isn't.
(There is an argument to be made that what are termed "indecent pseudo-photographs of children," which covers drawings, photoshops, 3D renderings, etc., of children which would be indecent if they were an actual photograph of that child in that situation, should not be criminalised because no child was abused or molested in its generation, but that is outside the scope of this writeup.)
This writeup also excludes so-called "extreme pornography," which was banned following the ill-informed carping of some woman called Liz Longhurst who blamed it for the death of her daughter, as opposed to the man who murdered her; apparently this stuff made him do it (do you believe that?)
But anyhow. The problem is, other than child porn, what is obscene? Good question. We know from past cases that Lady Chatterley's Lover is not obscene and indeed, it was under this Act that a prosecutor infamously said that it was not something one would want one's wife or servants to read. This didn't go down all that well. Similarly, Felix Dennis's infamous "Oz" magazine, and its "Schoolkids Oz" edition (in which among other things Rupert Bear got roundly and graphically buggered) was not obscene - well, they were convicted on first instance but then that was overturned at appeal, apocryphally when Lord Justice Widgery sent his clerk out to Soho to find the rankest filth he could and noted that the supposedly obscene Oz paled in comparison.
David Britton and Savoy Books and their 1991 novel "Lord Horror" was obscene, though. As was their "Meng and Ecker" series of comics. These, which were based in an alternate history in which Nazi Germany was victorious, attempted to explore how ordinary people can commit genocide - through the eyes of such a person. It particularly attracted the ire of Manchester's then Chief Constable, one James Anderton, or "God's Cop" as he was known. While an able Police chief who did get crime down, he was also a rampant homophobe and said that homosexuals were "swirling in a cesspit of their own making." In Lord Horror, one character reprises in full Anderton's speech about this but substitutes "Jews" for "homosexuals." This butthurt Chief Constable Anderton and Savoy Books were prosecuted for obscenity. I have only read Lord Horror in part, however, despite its pretence at exploring serious questions of moral responsibility - and its creator's assertion that he expected to go to prison for it because it was too subversive, it seems like just an excuse to write something squickful for its own sake. Michael Moorcock likes it, though.
More recently, moving into the internet era, a certain Stephen Perrin was convicted of obscenity over his website. He was running a porno site dedicated to coprophilia and contained such imagery. (As an aside, Russell Brand would approve. In 2005 he said that "if you find faecal matter involved in coitus, you do think, “What have we become?” Particularly after you’ve ejaculated." But that's probably because he's a vainglorious publicity hounding cunt.) This conviction was not overturned on appeal and as such it's pretty clear that in Britain, scat is obcene.
Later on, a Darryn Walker from South Tyneside was convicted of obscenity for writing a bad fan fiction called "Girls (Scream) Aloud" on his blog. In this, the five members of Girls Aloud shag each other in the tour bus while the driver listens in before he kidnaps them, forcing them to do unpleasant things to each other at knifepoint, then cutting them up and raping their corpses. As you do. Before selling the body parts on eBay. It's one of the worst things I've ever read, incidentally. However, the Internet Watch Foundation discovered it, reported it to Police, and he was prosecuted for obscenity. He was cleared when it was found that only people specifically looking for bad fanfics about necrophilia with popstars would see it and they are already depraved and corrupted. So, potentially this stuff would have been obscene if it had been more accessible.
The end result of all this is that legal advisers will say that people should take a precautionary approach, and self-censor. Because even if they are acquitted of any obscenity-related offence they can still be dragged through the mud. Darryn Walker was fired from his job and had a hard time finding alternative employment because of the controversy over his bad fanfic. But anyone who is minded to try and publish a boundary-pushing work for whatever reason will find themselves warned off this. There is also no doubt that OPA prosecutions, to my mind, can be politically motivated. Lord Horror being one, because it upset the powers that be, as did Lady Chatterley's Lover. Before the Acts came into force, Radcylffe Hall's thoroughly turgid novel of slightly implied lesbian passions, "The Well of Loneliness," was described as "disgusting." However, there is a slight hope spot, when in 2012 a certain Michael Peacock was found not guilty of obscenity even though his gay BDSM porn which he produced and/or distributed, between consenting adults, were not obscene. In many ways this was a further Lady Chatterley's Lover moment, as it recognised that BDSM is something that, well, is fairly mainstream. Indeed, one legal reported noted that jury members started to look bored as they were shown yet more alleged scenes of brutality and depravity.
Unfortunately I don't think it'll get very far, really. There's still the whole "extreme pornography" statutes to contend with. And the fact that obscenity seems to be entirely decided by the jury one gets on the day does not bode well. Not to mention the fact that people might not be willing to stake their freedom on the chance that they'll get 12 people who aren't upset by triple anal fisting or ultra-violence or general squick. To my mind, it's time we killed off the Obscene Publications Acts.