in a nutshell, a Q plate is issued if the vehicle has unknown origions, perhaps a stolen recoverd car that has had the vin ground off, and is therefore unidentifiable. the car would be issued a new vin number, and a Q plate.
some insurance companies inexplicably raise premiums for Q plate cars, although most yanks seem to escape this..
incidently, a large percentage of home built vehicles should be on Q plates, but people think that by screwing the vin plate from an old corsair or zephyr onto it, the vehicle becomes entightled to wear that reg. its not, and thus these cars are illegal, and unisurable.
in that respect, the Q isn't so bad, because at least you know the van is correctly regesterd and insured.