Test Channel 6 is an experimental television channel jointly operated by the BBC, ITV, Channel 4 and Five. These networks recruited a few hundred volunteer homes for an experimental technical trial to see how high-definition television could work on the Freeview digital service.
Currently, the channel is only available in London and the receiver must have a special Freeview set-top-box.
References
Test Channel 6 Wikipedia(Text) CC BY-SA