Nope, this is a professional-grade HF socket with a screw coupling - basically a couple of steps up from a BNC. About an inch wide with wicked-looking serrated edges. Can't remember the name of it offhand but it does have one.
Found the antenna eventually, where the removals men had conveniently taped it to the back of some shelf struts. No cable, no adapter - I assume it still exists, but no idea where any more. Tore apart a known-bad cable with an RCA at one end as a temporary solution. Found that my test machine was going to require some serious coaxing (haha) to convince it to output anything on the analogue output. Dug out BBC Micro - has BNC socket for composite video, not RCA - why? Dug out Spectrum +3 - has no composite output, only UHF.
Now looking for correct adapter for more modern computer. I'm not entirely sure that the 12-year-old Apple one I found is suitable for a PC video card, and it also looks suspiciously ratty for a cable that's never been used.
While tearing apart the spare room, found a lot of things that I'd previously been looking for (colorimeter, TrackIR, old Airport UFO, and my one good working 5.25" floppy drive) - directly underneath where the little old CRT had been. No wonder I couldn't find them.
Eventually found original cable I was looking for, in a completely different box at the opposite end of the flat to the rest of my radio gear. It changes BNC into the big socket, not RCA - time to dig out the Beeb again.
The key to knowledge is not to rely on people to teach you it.