ULEZ cameras aren’t speed cameras, they are ANPR (number plate recognition) cameras. They do not perform the same function. Speed cameras require the tools to measure speed over a set distance.
In addition, you probably saw a local speed watch initiative. Usually it’s specials with a volunteer for the community. You stand there for an hour and collect plates for anyone speeding. They will get a letter reminding them not to speed, not a fine, so it’s an awareness exercise, not a money-making one. The location is chosen from roads known to be hotpots for speeding.
Speed guns also have much longer ranges than cameras, so by the time you see the operator, they’ve already recorded your speed. The fact that they were standing near a speed camera (if it even was one, rather than an ANPR / ULEZ one) simply
means that it is a speeding hotspot.
Some poor woman was killed on a London bridge by a driver doing around 50 in a 20. She was crossing the road at a zebra crossing. If the driver had been doing 30, they believe she would have survived. Instead, he was more than double the limit and she is dead.
Speeding is a major issue and complaining that police resources are being used to save lives instead of protect property is wild.