There's a lot of really grand conceits in fandom, not the least of which is the urge to combine SF universes that don't have any connection with each other. It's a staple of fanfic - the TOS Enterprise meets Moonbase Alpha, or Dr. Who meets the (original) Galactica, or the Jupiter II visits The Seaview, or Quantum Leap bumps into Time Tunnel, or Kolchack turns up alongside Mulder and Scully. It's fun, it kind of writes itself, and of course you're not limited by copyright, budget, or the death of actors.
Fan Films are no different really. We've seen Kelvington's "Trek Through Time" in which The Doctor meets The Enterprise ( http://www.republibot.com/content/fan-film-friday-trek-through-time-kelv... )and we've seen Starfleet (After a fashion) meet up with Earthforce (After a fashion) here http://www.republibot.com/content/fan-film-friday-star-wreck-pirkinning and now we've got Space: 1999 knockin' boots with UFO.
Space: 1999 actually started out conceptually as a second season of UFO, but as time progressed, it became more and more separate, and by the time filming actually began, there was no relationship between the two series. There's a lot of similarities in style and feel, however, and a lot of the same faces were on board, both in front of and behind the camera. But it's not a spinoff, it's a standalone series.
Even so, fannon (As opposed to cannon) has generally insisted the two shows take place in the same universe, which makes it kind of inevitable that something like this would eventually happen:
Did I mention that it's a parody?
Details are a bit sketchy on this one, though it appears to have first turned up at a convention in the late 1990s. If you've got any information about it, or if you were connected to the production of it, drop me a line here firstname.lastname@example.org