Any question answered...

looks like a circular argument

It kind of is, by the DVLA. They wanted an easily read typeface which worked with simple metal embossing tools in the olden days, and concluded that they couldn't tell O from 0 without making the production process more complicated than it needed to be. Therefore, they came up with a registration mark allocation system which, while distinguishing between O and 0 for administrative purposes, never required the reader to make the distinction. If somebody told the cops they'd been run over by ABC12OA, the cops knew it was really ABC120A because ABC12OA is not a valid registration mark.

Forrins tend to use the slashed zero, but English people would not have seen those until computers started appearing in every office with dot matrix printers which had the same O/0 distinction problem.