How the two digits work
The numbers in the middle of a current-style plate are an age identifier, not a model year and not a serial number. A car registered between 1 March and 31 August gets the last two digits of that year. One registered between 1 September and the end of February gets the same number with 50 added, which is how you end up with a 77 and a 27 in the same year.
Adding 50 sounds arbitrary but it does a useful job. It keeps every identifier two digits long and makes the half of the year obvious at a glance, which matters more than it sounds when you are working out whether an advert's "2019 model" means March or September. The system started in September 2001 and has enough room to run to 2050.
Should you buy before or after a plate change?
The weeks before a change are the buyer's window. Dealers are clearing the current plate to make room, so there is more room to negotiate, and a car bought in late February or late August is mechanically identical to the one that lands a fortnight later. What it costs you is choice, and a plate that looks a step older as soon as the change happens.
Straight after a change the traffic runs the other way. Showrooms are busy, discounts are thinner, and the used market fills up with part exchanges, which is where the real bargains tend to be. Our guide to the best time to buy a used car goes through the whole year. Whichever way you go, the free MOT history check is the first thing to run on anything you are seriously considering.
Number plate years: the full table
Every age identifier issued so far, with its registration window and how old those cars are now. Each plate links to its own page, with MOT dates and the car tax system it sits under.