Doesn’t quite work like that Anita. AT is one of a number of prefixes assigned to Norwich. See this:
https://en.m.wikipedia.org/wiki/Vehicle_registration_plates_of_the_United_Kingdom
The prefixes tend to be used in order until the random letters are used up, so you might see AT on cars from three years ago but not coming round again for a while. You could check with a Norwich dealer what prefix is being used just now, you might get lucky.