Things you can’t mail to the UK:
—Citizens Band Radios, walkie-talkies, microbugs, and radio microphones that are capable of transmitting on any frequency between 26.1 and 29.7 megacycles per second and 88 to 108 Mhz per second.
—Goods made in foreign prisons, except those imported for a non-commercial purpose or of a kind not manufactured in the UK.
—Horror comics and matrices.

For more: http://geography.about.com/library/weekly/aa112300a.htm