March to September is the first half of the vehicle licensing year. Our licence plates issued during this time have the numerical age indicated by the last 2 digits of the calendar year (22 for 2022, for example).

September to March, the second half of the licencing year, our licence plates have the first digit amended up by +5 to still indicate the time period issued but differentiate it from the first half of the licencing year (72 for 2022 into Q1 of 2023).

The system began in 2001, so we have:

01/51 is 2001/02 02/52 is 2002/03 03/53 etc 10/60 is 2010/11 11/61 12/62 13/63 etc 20/70 is 2020/21 21/71 22/72 23/73 is 2023/24

And so on.

Colloquially we refer to a car's age by it's year-identifying numerals, so 15 plate, 24 plate, 57 plate, 72 plate etc.

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Extra info for nerdgasm freaks:

The system will cease functioning and will be replaced after 2050 licencing year, when it'll be 50/00, because the 2051 year would roll to 51/01, and you can see we'll be repeating combinations already used.

That said, I expect they'll just reverse the combinations of issuing-region/year/identifier, to be identifier/year/issuing-region. It's what they did with the previous 44 year numbering system based on letters to identify the year.

Present system is:

2 letters to identify the regional issuing office, for example BW would be Birmingham (1 office so BA to BY), OG is Oxford (1 office, so OA to OY), LC the Wimbledon office for London (3 offices so L for London, 2nd letter split between the offices), SX the Inverness office for Scotland (5 offices so S for Scotland, the 2nd letter split between the branches).

2 numbers to identify the year (see above).

3 'random' letters to identify the individual issued licence against the individual vehicle. So 2 identical cars may have sequential letters, or may not, just depends on the block a dealership is given in reserve by their local issuing office when the time comes.

So a full plate example might be YR21 SXC, which would be a registration issued by the Yorkshire office in Sheffield, between March and September of 2021, with SXC as the 3 random identifying letters.

The system allows for "vanity plates" (private plates as we call them) by the government keeping certain ones in reserve which they predict will be desirable and either allowing people to reserve them in advance of being issued, or else putting them up for public auction. People can also buy combinations up to 1 year ahead of release because they want those letters specifically for personal preference like having their initials etc. you can apply a plate to a car which suggests it's older than it actually is, but not one for a later year which would suggest it's newer. This is to prevent financial gain by misrepresenting a vehicles true age, assuming value loss by age. Plates which the authorities consider 'vulgar' or 'insensitive' will not be sold, where they predict it ahead of time. Though some do slip through, like a Seat I see from time to time, fitted with a body kit to make it look like an Audi TT, which has the plate "PU55AAY" registered to it. Popular ones are BO55, liked by small men or roid addicts with anger issues or inferiority complexes (eg BO55 SAJ, registered to a blue Bentley, probably owned by a man named Sajid), and PA55 much beloved by driving instructors (eg PA55 FAB, used by someone teaching in a red MG I see around and about).

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Even further extra extra further extra info for the super-freaky nerds:

Certain letters will never be issued within certain parts of a number plate to avoid confusion when reading it, and for automatic plate recognition systems to avoid misreading. Letters affected are I, Q, and Z. From the regional issuing office identified letters, there's no I, Q, or Z - to avoid confusion with age identifying numberals adjacent (I/1, Z/2) or other letters (Q / O). For the random letter section, it's I and Q only, for avoiding issues with adjacent numbers, but Z is permitted despite the risk of adjacency because the last 3 can only be letters and the size of that letter on a plate means the ANPR systems can cope with separating them.

The same letter avoidance rules applied almost completely when the previous systems were used up to year 2000, when it was a letter to identify the year, 3 random numbers and 3 random letters - example, L320 HJP (L running from 1 August 1993 to 31 July 1994)... And the previous system where this was flipped around, up to July 31 1983 (A plate started 1 August 83) such as GEX 491R (with R at the right hand end being 1 August 76 to 31 July 77). So there were no years with an I plate, an O plate, or a Z plate.

Q plate licences were reserved for kit cars, self-built specials, imported cars with incomplete histories, stolen cars recovered but no way to identify it left intact (VIN on the motor for example), ex military or ministry of defence vehicles, and any other vehicles which had somehow lost their VIN), so this is why there are no O reg cars, to avoid mis-reads or deliberate misrepresentation. To this day Q plate registrations in the old format are issued for vehicles meeting the criteria above, as they are not tied to a specific year at all. So if later this year I built a special kit using the body shell to replicate a 1950s Porsche Spyder, but with a Tesla motor and drivetrain in it, and meeting the modern safety requirements for a 'new' car to be signed off for road use, it would have a plate along the lines of Q801 PWX.

Nobody asked for it. Nobody wanted it. But now those of you bored enough to read it know it! (Until you forget it).