In the early 1990s, the BBC ran a short-lived service called BBC Select. It was designed to deliver highly specialist programming to narrow audiences. The programmes were broadcast after BBC One or BBC Two had stopped broadcasting for the day.
This example demonstrates the sort of thing BBC Select did. This is a programme about the Disability Working Allowance.
BBC Select was notable for using scrambled broadcasts. Anyone who wanted to receive BBC Select broadcasts had to buy a set-top box that would decode the signal and set your video cassette recorder to record it.
This video shows the scrambling in action. You need to fast forward to around 5:25 in this video. Alternatively, you can wait patiently through the four minute long ident — typically over-the-top for the 1990s!