An A plate/license sounds like a good idea, but as I've said before there are some problems that need to be overcome.
If the A is a permanent fixture of the plate, how do we prevent a non-A license holder in the same family driving the car and abusing the higher limit privilege? Equally, what about when an A license holder wants to use the car of another family member? Would a car with a non-removable A plate be a more attractive target for some thieving bastard who wants to cane it round the neighbourhood before setting it on fire, or would it still depend on what he knows how to steal without much effort? Anyhow, seems like they really need to be transferable from vehicle to vehicle.
But if it's to be a removable part of the plate how would we ensure that they aren't stolen for use by others who are not entitled to use them? Sure they can be taken off and kept in the house or whatever, but sooner or later somone will forget and leave it attached in the Marks and Sparks car park, and it won't be there when they get back. And if it's not attached properly the damn thing might drop off and end up on the side of the road to be picked up by the local boy racer (though intelligent design would probably sort that one out).
Next problem is enforcement. It would have to be hard enough to forge that a trafplod seeing one could be pretty certain it was the genuine articel. Also, it would have to be obvious enough that the trafplod could see it from a reasonable distance. You wouldn't want them tearing out of their observation laybys on the motorway after seeing a car doing 80 (or whatever), only to catch up and see the A plate that means they were actually allowed to drive that fast.
It would also mean a lot more work for the partnerships looking at Gatso pics to remove any A plated cars from the NIP pile. Hmmm. On reflection, who gives a shit about that? It wouldn't do any harm to force them to actually check the photos.

Might even stop daft NIPs going out for ancient Marinas breaking the sound barrier or whatever it was.
I'm not saying these couldn't all be overcome, or that I'm against the idea. But I do think that it's not a good substitute for intelligent enforcement at the discretion of a trained trafplod. I can't think how it would work in practice without using one of basingwerk's super dooper RF systems that let's the car know that an A license holder is driving so it can transmit that to the plods and scameras.....

what am I saying?
Bad Gatsobait! Bad!
Whack
Whack
Whack
Whack
