The whsmith question that you won't address is asking whether they wee right to choose to remove child porn books from sale.
Actually, I'll go there even if the estimable hamster won't. Yes, they were within their rights to do so, just as they were within their rights to (for decades) refuse to sell Private Eye. They're hardly consistent, though: for years they stocked and sold assorted BDSM "anonymous" novels from publishers like Silver Moon (absolutely not safe for work link). They can, however, sell or not sell whatever they and their shareholders want to sell or not sell, so long as what they sell is legal.
British law is in a real mess over textual depiction of illegal sexual acts, with the failure of the "Girls (Scream) Aloud" case to reach a conclusion meaning that it's not quite clear what the law actually is. It's unlikely that the CPS would touch such a prosecution now, and since 2009 endless websites containing material far more disturbing that in the GSA case (for example, those containing fictional textual depictions of sexual activities involving young children) are not on the IWF block list, nor is anyone proposing that they should be (and these are mainstream websites in the .com and .org domains, indexed by Google, not hooky DarkNet places).
But it's not entirely clear that an Obscene Publications Act prosecution would fail were it to be brought, WH Smith are rather higher profile than a random website, and are based in the UK. As they're making essentially no money from such sales, it's a reasonable commercial decision to decide that they don't want to take the risk.
So I think WHS were acting reasonably when they stopped selling those titles, as it's not obvious that an Obscene Publications Act prosecution would have failed, and the defences open to (say) Penguin over the Chatterley trial wouldn't have been available.
But there's absolutely no way that the book that is exercising people in this case would be the subject of an OSA prosecution, or any other credible legal threat. This is a taste issue, not a legality issue. So I'm not sure what the parallels that people are attempting to draw are. This is not borderline illegal material, no matter how much some people want to make it so.