as a hgv driver iwas on a industrial estate & stopped

I know this isn’t going to be the answer you want to hear, and it’s certainly not what I want to tell you. However, I wish to be completely honest with you, so I feel obligated to not give you false hope.
All that is a complete myth I’m afraid.
There is no obligation to highlight the presence of a speed detection device at all. They are perfectly free to high them behind overgrown trees and catch motorists by stealth in law. Speeding is a crime like any other. The police are under no obligation to warn people of their crime detection methods. In fairness, if test purchase officers had to warn people of their presence they would never catch any dealers and the different here is just one of degree unfortunately.
The ACPO guidelines do say that they should be highlighted. They are just guidelines and not law. Many of the Constabularies ignore them and there has been case law that weakens any need to enforce them.
What they cannot do in law is obscure the speed limit signs in any way. Then the motorist would have a challenge. There are only three automatic speed limits in the UK. The first is 30 mph in a restricted zone signed by street lights, the second is 60 mph in a single carriageway without street lights and the third is 70 mph on a motorway. If they want to vary those limits then they need authority from the Department of Transport and they need to erect signs that amount to ‘adequate guidance’ under S85 Road Traffic Act.
Sorry thats probably not the answer you wanted but it is the position that you have and I have a duty to give you truthful and accurate information even though its not what I want to say.