The Alpha moonbase sets and the Eagle ships and all the other "realistic" stuff was designed and built for the second season of
UFO. At the time
UFO was extremely successful, actually reaching #1 in the USA, I don't know how well it did in England.
Because the sets were designed for a secret advanced government organization in the present day, they all appear to be near-future equipment and a high degree of realism.
But the show certainly went far into the fantasy realm, and even human technology introduced in it appeared to be centuries advanced over the appearance of the sets.
UFO was unexpectedly cancelled (the story I heard is that the American TV network moved it against Dallas and it suddenly bombed). This does not explain why the powers that cancelled UFO were willing to fund a similar and just as expensive show by exactly the same people, but the sets and creators were all reused with a new plot and that is where Space 1999 came from.