The story begins when I was first licensed in 1979, I took, and passed the City and Guilds 765 Radio Amateurs Exam parts 1 and 2 (the RAE). At the time there was only one examination unlike the three of today, the two parts were licensing conditions and radio theory, and you had to pass both to get a license. Following an exam pass there were two options, you could apply for a "B" license (G8*** callsign) which permitted operation on the VHF bands upwards (note; 70Mhz was the lowest available band as 50Mhz was not allowed in the UK then). Alternatively you could take a Post Office 12wpm Morse Test at a Coastal radio station, then apply for your "A" license (G4*** callsign) which gave access to all bands.
Apart from access to new bands, 10Mhz, 18Mhz, 24Mhz and 50Mhz not much changed for a few years until the introduction of the Novice License (later to be re-named Intermediate). A simpler exam but with the practical addition of some radio based construction gained the novice licensee with 50 watts, again with "A" and "B" variants depending on success in a 5wpm Morse test.
In pre-empting the international requirement of a Morse test pass to allow access to the HF bands being abolished, the Radio Licensing Agency decided in the late 1990's to introduce a class "AB" license, which gave those who had passed the RAE, and successfully completed a 5wpm Morse test access to the HF bands with a power limit of 100 watts rather than the 400 watts of the "A" license. In 2000 I took the 5wpm Morse test at York Amateur Radio Rally and was then able to apply for my "AB" license getting the callsign M5TXJ. Not long later the international Morse requirement was dropped so "A" and "B" licensees gained HF access with their original calls and new full licensees got M0 callsigns issued as all were effectively "A"'s. With this, M5's were discontinued, I believe there are approximately 240 of us, a number slowly reducing. All M5's were given the option of swapping for an M0 callsign, I prefer to keep my exclusivity!