-Here's the lowdown on "ALTERNATIVE 3",from a TV-movie compendium:

"ALTERNATIVE 3"

(GB 1977; 52m, colour)
Amusing spoof do commentary about the disappearance of various high-IQ
citizens, allegedly to form nucleus of a standby civilization on Mars against
the coming End of the World.  Sly parodies of fashionable breathless TV
journalism sweetened the joke, ex- newscaster Tim Brinton held it all
together with po-faced gravity and needless to say some supernature fanatics
refuse to this day to accept that it was anything but gospel truth, although
it was orignally scheduled for April 1st (1977).

Written by David Ambrose;
Directed by Chris Miles; for Anglia.
Apparently the TV-movie was spawned by a book (or assuming the date is accurate, vice versa) of the same name.

Written by Leslie Watkins, it was published by Sphere Books Ltd. in 1978.