In AutoExpress they responded to a question asking about black circles rather than red on speed limit signs:
AutoExpress wrote:
Question: I have noticed that on some roads there are speed limit signs featuring a number in a black circle, rather that a red one. Do they serve a purpose?
Answer: These signs are being put up across the UK and have been designed to warn about what's going to happen further along a road. The black-edged boards are situated before a slower speed limit takes effect - where the red-edged signs are located - to warn a change is about to happen, and are therefore advisory. You're most likely to find them before a blind bend, after which you'll be required to brake immediately.
This of course is just incompetance by the DfT. When I drove around Edinburgh many years ago they had a very effective method for doing this - that was obvious to me even though I had never seen it before - they have a 300yd, 200yd & 100 yd countdown before the limit starts. So why come up with another method that gives less information, and which is deliberately confusing and not advertised to the driving public?
I have no idea where AutoExpress got their information from - they are not covered in the highway code, and I cannot google anything that describes them, so the sole purpose appears to be to cause confusion and to clutter up the road with more pointless distractions.