Any material judged to be obscene under the
current interpretation of the Obscene Publications
Act 1959 (could include bestiality, bondage, rape simulation, etc.) People have even been charged for writing stories, though I don't think anyone has ever been successfully prosecuted (https://en.wikipedia.org/wiki/R_v_Walker)
How on earth can that act be deemed as this government banning 'some forms of pornography'? Even R v Walker is 2009 - a year before even the last government. Although UK production and distribution of some acts were banned last year, the pre-Coalition governments were far more draconian with their criminalisation of certain material involving consenting adults.