The exact system has changed over time. Prior to 1963, there was no year coded into the number at all, and numbers just appeared as something like ABC 123 (although the BC part of that did tell you
wher it was registered originally).
The first few suffix letters were based on the calendar year of when the car was registered: A=1963, B=1964, etc. However from E/F onward, the system changed so that new registrations came out in August, so F was August 1967 through July 1968, and so on.
The August change continued when the year letter became the prefix, so A = August 1983 through July 1984, and so on.
However, the last few prefix letters were changed every six months in order to "use them up" before the completely new coding system was introduced for 2001 models onward. A "T-reg" (as prefix) means that the car was
registered between March and August 1999.
This link has a full list of all suffix and prefix letters, starting about two-thirds of the way down the page:
http://www.wolfbane.com/vreg.htm(Lots of other information on the area codes as well.)