Many European countries use two or three-letter abbreviations of the local licensing authority, but of course the simplest and most logical system belongs to the French.
Every number plate contains two digits (up to 99 representing the Mining and Mineralogical District where the car was registered, Paris is 73). Other numbers and letters can be placed on both sides, and as far as I can see the system has not yet run out. The heavy hand of administration had afflicted French motorists even by 1899, and before a car or its driver were allowed on the road they had to be inspected by the local Engineer of Mines.
I appreciate that our registration letters are regarded with affection and nostalgia by many, but surely the end of the present year-prefix means that there can be no further incarnation. Might I suggest that we have a ready made system as used in Europe: the postcodes which cover the whole country which, combined with other characters and even the beloved year-letter, could carry us well into the next century.