Thanks for the explanation, I can see it does strictly make sense.

However, given how few people use post towns in addressing any more (since their purpose is fully covered by the first half of the postcode), and how the ID editor doesn’t provide any guidance about esoteric UK addressing rules, you might be fighting a losing battle to change all addresses to use post towns.

I don’t mean to be negative, but would it be a more productive use of time to ensure that postcodes are present (and correct) everywhere in OSM in the UK? They convey more information than the post town, fit in nicely with the ID editor defaults, and are more understood by people.