Not just instructors though. I had a conversation with a traffic cop a few years ago and he said that if he pulled someone for speeding and asked if they knew what the limit was and if they knew what speed they were doing (within a reasonable margin) and they were wrong or said that they did not know then it was sufficient grounds for a charge under Section 3 of the Road Traffic Act 1988.She would say that wouldn't she.
Interestingly on my Zoe's TomTom maps it shows the speed limits on most roads. I have a 14 mile commute and there are two stretches of road on just that which are wrong. One shows 60 where it is actually 40 (and has been 40 for around 25 years since they started developing the housing here) and one showing 70 where it is actually 30 - on a road which is only about 20 years old, has always been 30 and is a notorious speed trap. Beyond those two I've found it generally reliable.