I wasn't born before 63 so I'm not sure if there was a sequence then but any combination of (up to) 3 letters and (up to) 3 numbers could be had from A1 to YYY 999
In normal form, starting 1963, UK registration plates had a letter of the alphabet that was assigned to year of registration.
So, back in August 63 if you bought a car and didn't want any particular registration it would be assigned one by the DVLA and take the form of three letters, space, three numbers, space and the letter A (for 63).
In August 64 the last letter changed to B and so on.
Just to confuse matters, the letters I, O and Z were omitted from this sequence so Y was the last to take this format in 1982.
Then the format was reversed so the year letter would be at the beginning (so starting with A in August 82).
Then when that sequence was also used up they introduced 6 month intervals instead of yearly and changed the format completely to letter, letter, number, number, letter, letter, letter where the two numbers now represented a 6 month period. For some reason, it was decided that this two number sequence should start with 02.
So, (are you bored yet?) it sometimes happened that the plate that was assigned might look like a word or phrase and so cause amusement or derision, depending upon the viewpiont of the observer.
Certain combinations might confer some special status such as HRH 1.
Rumour has it that a wealthy individual residing in one of the more salubrious parts of London had two Ferraris with the registration numbers "NOT 1" and "BUT 2" which he liked to park side by side.....