The UK's Obscene Publications Act of 1959 and 1964 lays out the law governing what can and can't be shown in the UK. Something is deemed to be obscene if "its effect is such as to tend to deprave and corrupt persons who are likely, having regard to all relevant circumstances, to read, see or hear the matter contained or embodied in it."
The British Legislature is infamous for such wooly statements (The Regulation of Investigatory Powers Bill (UK) is the latest in legal obfuscation). Most police define obscene (with specific regard to pornography) as any material showing an erect penis or any kind of penetration. This makes watching porn in the UK a rather... disjointed experience