It is interesting, the dynamic currently.
I think with the camera/database and automated method of checking number plates, they already know what kind of driver you are (speeding fines, previous defects, how often you have been to the toilet today, etc.) Any flags and they go for it.
So they already know you and your car.
Otherwise, it's probably discretionary..... "Quiet day Sergeant isn't it? Yep, OH, there's a Datsun 1600, used to have one of them...let's pull 'em over and see what's under the bonnet...."
"Cheeesuss... Must go alright?....."
"Seems like a nice respectful bloke, let's give him a random breath test and let him go..."
Is usually what I get.