Re: And this ladies and gentlemen...
https://www.autoexpress.co.uk/car-news/96260/uk-car-number-plates-explained-rules-history-and-what-they-mean
States that O isn't used, but Wikipedia says that O is used for Oxford area. There was a change around 2001. Five minutes of digging hasn't brpught up anything definitive - perhaps you guys can do better.
Ultimately, the whole damned point of a licence plate is that someone can report that a "red car, TF20 something something possibly a P" has just sped off after running over their dog - and understandably they won't have the best view of the plate or the best state of mind to remember it. The first part of the plate is the area code on the grounds that witnesses are more likely to read it, and knowing the area makes searching for 'red car's easier - especially in the days when different areas had different databases.