I honestly would not stop if I had a p-plater behind me with flashing lights, its just way too risky. They can quite happily follow me back to the nearest police station but I aint stopping.
Just like about a month ago I seen an undercover copper in a toyota tarago pull over a car on the M4 Westbound, like hell im stopping for a tarago with some flashing lights on their dashboard.
It really wouldn't be that hard to hijack a car these days. Throw some flashing lights in my car and find another nice looking car, turn on the lights, pull over the car and me and a mate get out with a baseball bat, flatten the driver and steal the persons car. So why the heck should I take that risk with a undercover p-plater?