I don’t know if a warning notice is a legal requirement. I have only once ever been stopped for allegedly speeding: It was in Spain.
There was no warning sign and as it happened the officer with the radar gun, who had a radio link with the officer down the road who was there to stop and report, had some communications problems in that the signal was not clear, When I disputed the claim that I was speeding the officer called the one with the radar gun to come to the scene. When he did he said it was not me who was speeding but another white car. The officer apologised for stopping me, saluted, and I went on my way.
An aside. I was in the canteen at Bow Street police station some years ago. A traffic control officer had left a radar gun on a table. A waitress accidentally knocked it off the table. The officer picked up the gun; his colleague pushed a trolley used for collecting dirty plate to test the gun. It showed he was doing 3 mph.
NB if more than 30 years ago a radar gun could detect the speed of a canteen trolley, don’t think that in any vehicle you may get away with speeding.