I've just been reading some more about these BBC encrypted services.
I believe that they came about because the then Thatcher Government told the BBC to utilise their spare capacity to raise revenue as opposed to asking for repeated licence fee increases.
It started with doctors, but was then extended to include dentists, nurses & car mechanics.
This was a commercial failure and was relaunched as BBC Select in the early nineties:
https://en.m.wikipedia.org/wiki/BBC_Select
I remember accidentally coming across some of these tests late one night.
Apparently, so did other people who were genuinely concerned that aliens were trying to contact us! Don't forget that at this time there had never been any scrambled TV signals in the UK*.
* Apart from analogue cable.
Eamonn Holmes had to reassure viewers the following morning in his live 'Open Air' programme
For those who haven't a clue what I'm talking about
:
https://m.youtube.com/watch?v=89fW2gum-B0
https://m.youtube.com/watch?v=lRFAFsEg0MI
https://m.youtube.com/watch?v=uisMEnYM2nM