The RF-Man site provides a list showing the year in which each UK amateur radio prefix was first issued as well as a history of CB radio

Up until April 1, 2000, amateur callsigns were issued sequentially which meant you could tell when someone was issued with their licence. After that date callsigns could be selected from any available call within the current class block.

After 2017 it became possible to request a callsign from any of the previously issued call blocks. This meant someone passing their Full exam could apply for a callsign from among others the G2, G3, G4, G5, M5 prefix call blocks.

See the UK Calls page at  
https://ukspec.tripod.com/rf/ukcalls.html

GD0OUD's site also provides a page on the history of CB Radio, it notes in the UK from November 1, 1999, all CB licences were issued with Callsigns, these were in the format "2 + letter + digit + letter + letter" e.g. 2A1BC. This continued until CB was made licence exempt on December 8, 2006, see
https://ukspec.tripod.com/rf/cb/