From the 1930s, saucy cartoons were increasingly popular until Donald McGill, famed for his buxom beach babes (see above), was prosecuted in 1954 for breaking the ObscenePublications Act.
He said the former was "over an allegation or allegations relating to indecent obscenepublications" and the latter "over a claim or claims relating to the Public Bodies Corrupt Practices Act 1889".
While an R18 rating is not a legal defence for a pornographer, it would be very unlikely for the ObscenePublications Squad to take action over a film passed by the BBFC.