"Death's Door," listed on Wikipedia as "the eighteenth episode of the fifth series of the 1960s cult British spy-fi television series The Avengers." (In the USA, that would be called "the fifth season" of an ongoing show, instead of being labeled as a separate "series," and this particular episode's number might be abbreviated as S05E18.) Throughout the show's run, Patrick Macnee played the role of Mr. John Steed, working for some sort of intelligence agency within the British government, and this is one of the episodes which co-starred Diana Rigg as Mrs. Emma Peel, who assisted Steed in his discreet investigations of various things which had aroused the concern of his superiors. (Macnee also had various other female co-stars, both before and after her tenure, but she is the one who is best-remembered, and I suspect her episodes are the ones most likely to get reruns in one country or another.)
As you remembered: There were various science fiction tropes explored during the show, including unrealistic inventions, and often mysterious plot points which were at least alleged to be caused by "ghosts," "aliens from outer space," "time travel," etc., although sometimes those things turned out to have been contrived as cover stories for something else which had a more "realistic" explanation. (On the other hand, I remember well an episode in which we saw that John Steed's and Emma Peel's minds really were swapped with those of a couple of Russian spies, each ending up in someone else's body, although their British superiors absolutely refused to believe it when our heroes tried to report what had happened. That's one example of how fanciful the show could get.)
This episode was particularly surreal. The basic plot premise is that some villains are going to great lengths to delay whatever the British government is planning to announce in a big international peace conference being hosted in London. A man named Sir Andrew Boyd was originally assigned to be the delegate speaking on behalf of the United Kingdom. At the beginning of the episode, he panics for some reason, and suddenly refuses to enter the room where he was supposed to speak to the conference. Mr. Steed and Mrs. Peel are assigned to escort him safely to the conference tomorrow morning to try again. I'll now quote a bit of the plot summary from Wikipedia, and you can see if it matches your recollections.
The following day on the way to the conference, Boyd begins predicting
events, from the button missing on the coat of the butler, and
mentioning that he will see a lion before his death. Arriving at the
rescheduled conference, Boyd again becomes disoriented but flees this
time even before he enters the building and in doing so is killed by a
passing car. The last thing he sees is a lion's head sculpture on the
wall of the path approaching the building.
Boyd is replaced by the younger Lord Melford who promises none of the
nonsense that has just occurred. However, that night he has a
nightmare, including seeing 12 o'clock on a clock, his bathroom
cabinet collapsing, a Friday the 13th calendar, a cut on the face of
an associate, a broken-down elevator, men dropping a box when getting
out of a truck, a cyclist being run down by his car, a handle coming
off the briefcase, seeing a sinister looking foreigner before the
Conference door, and finally seeing a large chandelier falling upon
his head, killing him. The following day, every turn of events in his
dream starts to come true to the point that as he approaches the
conference room door and, like Boyd, he refuses to enter and leaves.
He experiences a similar dream the following night and informs Steed
and is so certain of his premonition that he will not attend the
To skip ahead a bit: After Lord Melford has described his frighteningly vivid dreams to Steed and Peel, they begin investigating the possibilities for how certain things could have been arranged to happen in accordance with what he saw and heard in dreams. They are working on the assumption that something other than psychic precognition is taking place. While they are pursuing separate leads, this happens:
Peel meanwhile investigates the broken down elevator and finds it has
been tampered with and traces it, subduing the culprit and finding a
tag with the same warehouse address. Peel arrives at the warehouse
first and discovers that all of the items experienced in the delegates
dreams are in fact reality and discovers that the warehouse contains
nothing but props seen in the dreams, including a mock conference room
and door. It appears that the delegates were drugged and brought to
the warehouse in their sleep and programmed to scare them away from
the conference by the perpetrators to delay it for political reasons.
Peel and Steed bring Lord Melford to the warehouse and as they
contemplate the situation they realize that the associate with the
plaster over the cut on his face must be in on the act. They meet him
just as he is leaving the abandoned conference and a struggle ensues
and the chandelier in the room is weakened by a stray gunshot during
the fight and ironically, the chandelier falls and kills the man who
had dreamed up the scheme.
As you suggested in your post, it's hard to untangle the motives of the villains. Did they really think they could keep this up indefinitely to prevent the British government from going ahead with whatever big policy decision had already been made at the highest levels? But the producers of The Avengers were often less concerned with realism than with putting on colorful and intriguing spectacles, and I thought they did a good job of it in this episode.