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I added the answer on Flickr there, it was a fingerpost which were exempt from removal.
I have since found a handful of old warning signs around London, one actually does have a triangle so sorted on the main front, although a corner has dropped of the old school sign in Bethnal Green. I hear they are making the road one way and those tend to be the times any old signs are discovered and taken as they did here when they put humps in (although not when they extended the kerb a few years earlier).
I just found a Halt sign of my own on a private road in Bushey, and one I don't remember seeing 'at main road ahead' as it wasn't that major. Like the T junction also on a private road in Hertford it had no circle as they usually did, but the sign was in very good shape.
That leaves three known working UK halt signs as another turned up in Bristol and sharp eyes around the country are gradually filling in more with the lion's share around Birmingham and a few around Glastonbury. With the internet now it's possible to find many locally someone else has posted until we catch up with many more, probably with the help of Streetview now as well.