Perhaps the white car plates you were referring to are cars that have been manufactured before 1973. Cars made before this year are allowed to use number plates that are colored black and white. However, most part of it are black rather than white.
Another probability that will answer your inquiry is that the car you saw may be from the Crown Dependencies of the UK, mostly those islets located in the Channel Islands and the Isle of Man.
In many countries, number plates are allowed to be customized for a great sum of fee. The yellow and the white plates may be UK's way of customizing car plates.
SOURCE: http://en.wikipedia.org/wiki/British_car_number_plates