That had been puzzling me too, in those idle hours of ... What were we talking about? Oh, yes, serials.
A mate told me there's a Committee at the Air Box looking into it. They are taking advice from BT, who have proven skills in making numbering sequences completely incomprehensible, and DVLA who live on another planet but are experts in irrelevant letters and numbers. Accordingly the results won't be known until 2019
I see two options ... Either AAAnnn or nnnAA. Or hexadecimal, starting at 0.
(Oops, that's 3 options, or 10 in Hex IIRC ... Or is that binary?)