PLOD may not have asked for your details as he had them from the PNC!!
They have to give you a warning first, second time a fine can be given third time they withdraw the reg, and physically unscrew the plates there and then, then they can if they wanted to get it towed at your expense and charge you storage until you can get it back when you have your new reg number, just not worth the hassle.
I have short number plates but mine legal as I have two number ones so cut down so they have the 11mm at the edges still, look nice but legal.
tabetha