They used to do this, I got pulled over many times for no real reason in the first few years of starting driving. It doesn't seem to happen now, maybe it's all part of the stop and search/human rights stuff.
Given the amount of cars on London's famous potholey roads, how many times in the last 6 months have you seen police pulling over a vehicle? think hard. It's a small number isn't it? maybe even a zero.
It's not even hard, 1 in 3 cars that passes will be doing something wrong, they need no special excuse to pull someone over, so yes, why isn't it happening?
When was the last time you saw a traffic officer?