Off I go ranting about witheld reg and I've not yet given you the low down on what number plate formats there actually are yet.
There's 5 main types of registration.
The first one is termed a prefix plate. You're probably familiar with these. A single letter - which is an identifier of the age of the vehicle - a number (1, 2 or 3 digits), and three more letters. Something like A567 TRD. They're called prefix plates as it's the prefix letter that discloses the age of the vehicle. Incase you hadn't twigged.
These ran from 1983 through to 2001. Starting with A reg, and ending in Y. There were never any I, O, U, Q or Z prefix letters. I's look too much like the number 1. O's like a zero. U's looked a bit too much like a V. Z was never a letter used in British number plates (see more on this below) and the Q was used on "special case" vehicles.
You might've spotted a Q reg on the road at somepoint. Essentially these reg are given to vehicles where their age is undetermined. Some imports, cars that have been stolen and recovered, kit cars, cut and shuts - things like that. They're not part of general use and can't be transferred to or from these vehicles. They're stuck with them for their lives.
Prior to the prefix format, we had the suffix format. Three letters, a number (1, 2 or 3 digits) and a year identifying letter. Amazingly enough so called because the year identifier was the last (suffix) letter. No duh.
This ran from 1963 to 1983 when the prefix format took over. Like the prefix system, it started at A and ran through the alphabet to Y, skipping the same special letters. An example reg would be TRD 567A.
Prior to that there was just chaos. Cars were assigned numbers based on their regional location, what the county council felt like that day, and what the weather was like. I'm sure there was SOME kind of pattern, but the reg carried no discernable year identifiers and so are termed dateless reg.
Dateless reg cover the likes of your classic A 1. They can have one, two or three letters, followed by 1, 2 or 3 digits. Or the other way around. 1, 2 or 3 digits followed by 1, 2 or 3 letters. So for every A 1, there'd be a 1 A. These pre-date 1963 and go back to the start of the 20th century, so carry a lot of history with them. The sad thing about these reg is that many will simply have faded away with the vehicles they were on when they were scrapped. It's only a handful of foresighted individuals who've managed to retain their reg and pass them along that can offer them for sale today.
Q, Z and I are letters excluded from these dateless reg.
Our fourth type of registration is the Irish reg. You can use Irish issued registrations on English/Scottish/Welsh registered cars. These reg are very similar to our classic dateless registrations, but ALWAYS contain an I or a Z in their letter combinations which makes them identifiable. 2 or 3 letters followed by 1, 2, 3 or 4 digits or the reverse (1, 2, 3 or 4 digits then 2 or 3 letters). These are great if you're lucky enough to have a name like BAZ or GAZ. The order in which they're released is a little cryptic (as our dateless reg were), but basically in Ireland they'll pick a 3 letter sequence - e.g OIL, and issue all the reg from OIL 1 to OIL 9999, and when they run out, pick another three letters and start again.
The fifth and final type of number plate you'll see on UK roads is the new style which came into play in 2001. Sometime reffered to as millennium reg because of this.
When the prefix system ran out of combinations this new format was introduced to take over. rather than rely on a letter as a year identifier (as with the prefix & suffix formats), the system works using a number to identify the age of the vehicle it's on. First off, in late 2001, we had 51. The 5 signifying the second half of the year, the 1 representing 2001. In early 2002 we had 02 reg. Followed by 52. Then 03, 53, 04, 54, 05, 55 and so on. Eventually we'll work our way up to 99 in 2049 and they'll have to come up with another new system, but that's some 45 years away yet.
The rest of the reg is made up of 2 letters, the identifying number (which is always 2 digits), and three more letters. The front 2 represent the area of the country the car was registered in, the last three are just random letters - e.g AB51 GHV.
For the first time, the letter Z was allowed in these reg BUT only in the last three letters. So you can now pick up GAZ and BAZ plates such as AA51 BAZ (£699) that simply weren't possible combinations outside of Ireland until now.
And that is your lot.