i think and i;m sure gilles will correct me if i'm wrong' but the last 2 digits of the froggie rego declare where the car is from. i.e 1234AB72 would be from the le mans region. pointless info i know.
and the pair of...
... letters are chronologiccaly attribuated.
Maybe your example of 1234 AB 72 is a car registered in the early 50's.
My '63 panhard is 766 GJ 72 and so first registered in december 63
You hardly choose your licence plate, it's an ordinal and chronological automatic attribution.
When you change your car of department (example if you move or if the new seller is not living in the same department), you'll need to have a new number.
It explain why so much classic cars don't have any more their original licence plates (mine has got them)
But from 2007 all cars in EU will need to change their plate and every car will have one and only number, no matter it has been sold in an another foreign country within the EU.
Coming to this point: how much for a little resort on the southern coast, almost just to have a UK adress and being able to own artisanal cars ??