This isn't a K8, it's some strange plastic hybrid kiosk. Perhaps used as a moveable prop for filming? A pair of them, with the smaller white telephone sign at the top and the strange plastic panel at the bottom can be seen in episode 1 of `Smiley's People', the 1982 BBC spy drama series sequel to Tinker Tailor. Anyone have any information on it?
The description was taken directly from BT, so I'm afraid if it's incorrect then that's up to them. I agree it does look a bit odd, at first I thought that could just be because the video had been stretched (or some similar unwanted image manipulation), but on closer inspection the telephone sign is definitely the wrong size. It's interesting that it appears in other places, let me know if you find out more!