Thing is, going back to the OP, Fantastic Journey, Man from Atlantis and Invisible Man all got run on UK TV, and Fantastic Journey ran about three times on the BBC (mostly in the mornings during school holidays). So I'd guess that anyone in the UK of about 40 to 45 with any sort of childhood interest in SF probably remembers that a lot better than many of the non-hit SF series of the last 20-odd years. Even though they'd be hard pressed to name any of the characters ("There was the guy from the future, the sneaky treacherous doctor, the girl from Atlantis who had a cat but kept on disappearing from one episode to another... oh, and the kid. The one who got killed in that Trek film. the really good one.")
Oh, and Christopher: adding another couple of British kids SF series to your multi-season list - Into the Labyrinth (included episodes by Robert Holmes, Bob Baker and Christopher Priest) and The Tomorrow People.