Prior to 1962, several systems were used. A1 was the first number plate, and other systems gave numbers like 1L, E77, DF1951, FAB123, 1802MF and so on.
From 1962 to 1983, the UK vehicle number plate looked like this:
ABC 123D
...where the A was random or sequential, the BC was an area related code, the 123 was random or sequential and the D was the year letter. A was 1963, B was from January 1964 and so on. In 1967 the change was moved to August: E came in at Jan 1967 and F at Aug 1967. (thanks stupot).
The letters I, Q, and Z weren't used in the ABC section, and I, O, Q, U and Z were not used as year letters (although I and Z were valid in Eire). Q denoted a special registration such as a kit car or an imported vehicle of non-provable age.
In 1983, the system had reached Y-registration, and had run out of letters. A new system was introduced, which simply changed the position of the three letters and the year letter:
A123 BCD
This continued in the same vein as the old scheme until 1999, when six-monthly letter changes were introduced. S was August 1998, T was March 1999, and from then on, changes occurred every March and September.
Yesterday (1 March 2001), the Y date letter was issued. That's the end of another system.
As of September 2001, the replacement system will look like this:
AB51 CDE
The AB is the regional code, although a different code to the old system. The two numbers are a date code: March 20XX will get XX, September 20XX will get XX+50, so this system works until 2049. The CDE bit is randomly or sequentially issued. This time around, Z will be a valid letter.
See http://www.wolfbane.com/vreg.htm for more details including regional codes.