I like UFO the best... one of my favorites. Though apparently the idea that it was intended to be an S:1999 spinoff is actually a myth.
It's the other way.
A second season for UFO (iirc) would have seen a much enlarged moonbase with Shado HQ moved from under the film studio to the lunar surface and taking place 20 years later.
However the ratings dropped towards the end of the run so a second season was shelved but a lot of design work had been done and Anderson was determined to do something with it.