The information on this 1964 BBC production is limited, but from the little surviving footage it is - at least to me - quite obvious that it is indeed a great loss for every science fiction fan.

Thanks to wikipedia we do at least have some basic facts:

"An adaptation of The Caves of Steel was made by the BBC and broadcast on BBC2 on 5 June 1964 as part of an anthology strand called Story Parade, which specialized in adaptations of modern novels. It starred Peter Cushing as Elijah Baley and John Carson as R. Daneel Olivaw. The adaptation was the brainchild of Story Parade story editor Irene Shubik, who was an enthusiast of science fiction and a fan of Isaac Asimov in particular, once referring to him as “one of the most interesting and amusing men I have ever met”. [...]

The screenplay was generally faithful to the plot of the novel. The only major deviation was the conclusion – in the television version the murderer commits suicide when he is unmasked, although in the novel he agrees to work to convince the Medievalists to change their ways. The other major change is that the roboticist Dr. Gerrigal is a female character in the television version. [...]
The play was repeated on BBC1 on 28 August 1964. As was common practice at the time, the master tapes of The Caves of Steel were wiped some time after broadcast and the play remains missing to this day. A few short extracts survive: the opening titles and the murder of Sarton; Elijah and Daneel meeting Dr. Gerrigel (Naomi Chance) and Elijah and Daneel confronting the Medievalist Clousarr (John Boyd-Brent)."

Some more information about the production can be found on this page.

Needless to say: If anybody has any information about a copy of that play, I am highly interested in hearing about it.