As I understand it, most UK broadcasting is now covered by the
Communications Act 2003 which came into force on 1 April 2004. Certain parts of this act are further defined by the
Statutory Instrument 2004 No 692. I'm not sure if this replaces the Cable and Broadcasting Act 1984 but, as spiderplant mentions, this is not available online.
The
must-carry obligations are defined in section 64 of the 2003 act.
As to what it all means - well I could only stab a guess. Perhaps some of our legal friends could make things clearer.