Meeting the noise regs and acceptable noise levels / responsible driving in town / city streets are two very different things.
I'm not sure using the defense of "The car has been certified to meet noise levels" whilst driving through town WOT in 1st, is a good defense.
I imagine any stock car driven responsibly for the surrounding wouldn't result in the driver getting a fine.
I guess it will really depend how sensitive said cameras are, classic cars could for sure be at risk especially the old 2/3 speed automatics, but generally I agree most cars driven in a city as per how you would expect a car to be driven in a city should not be breaking the sound limits. I guess it will also catch all the people driving around in a lot of unroadworthy stuff with broken exhaust as well.
Me personally it is likely never going to impact I rarely visit a city and if I am going into a city I will generally take the Yaris.
They should put these noise meters in London, their is a huge collection of super and hyper cars there, all running de-cats making an absolute racket, so if they want to earn plenty of money they will.