As here, Quentin:
In the past (pre '62) the Brits used to, simplistically, look at the year of registration as being the value guide on a used vehicle.
Hence, to get your full money's worth, you should take delivery of your new car on January 1st. This caused a very uneven distribution of sales, month by month, posing problems for manufacturers and car sales outlets.
The "solution" was to have an annual registration letter suffix, applied from 1st August. 'A', in 1963, 'B', in '64, etc.
When they ran out of suffix letters, in '82, they started prefix lettering 'A', in '83, etc.
That ran out in 2001. At which point, because the problem of the market being concentrated in year and letter, they went to a half-arsed number system, twice a year in March and August.
The non-intellectually challenged among us, admittedly a minority, would look at a vehicle's mileage, condition, service history, etc., and not imagine that a vehicle first registered on 31st December of one year was one year older than a similar vehicle first registered on 1st January of the next year.
As the great HL Mencken wrote, in 1926:
"No one in this world, so far as I know—and I have searched the records for years, and employed agents to help me—has ever lost money by underestimating the intelligence of the great masses of the plain people."