The Obscene Publications Act covers broadcasting as well as print publications. This makes it illegal to 'publish' (i.e. broadcast) any material which is likely to corrupt or deprave the viewer.
The Protection of Children Act (as amended by the Criminal Justice Act and the Sexual Offences Act) makes it illegal to transmit indecent images of children.
The legislation relating to libel apply to both the words and images transmitted by TV stations.
The law against blasphemy provides some protection to the Christian religion against certain types of material being transmitted.
The Representation of the People Act ensures that television stations don't give a particular advantage to any candidate (thus disadvantaging his/her opponents).
Lastly, the Communications Act 2003 gives authority to Ofcom to oversee the day-to-day regulation of British broadcasting.
Course, there's probably a regulation outlining the need for an 'Off' button, as all tvs seem to have them. However, most who complain about tv output haven't read that they exist, apparently.