UK drivers, can you tell if a car in front or you walk past, has dash cam, and if yes then would you think anything about it?

If the car is front of you you'd likely not notice it.

If I did see a camera in the windscreen of a car, I'd think nothing of it. In fact I often travel as a passenger with a friend who does have such a camera in his car.

I've not fitted one, but have fitted a reversing-camera. That's different though, and switched on only to help me park in a tight spot.