I'm from Ireland, where filling beers precisely up to the brim is practically a religion, & many barmen will even take the glass back & top it up if they see the head diminishing too quickly in the space of time it takes you to pick the freshly poured pint up.

One thing that always struck me as odd is how the culture is seemingly the opposite of this in apparent beer meccas like Belgium - not only are the glasses typically much smaller (this is fine) but they also leave massive gaps at the top. The glass capacity is never treated as being close to the rim at all.

Kind of.

Guinness glasses are exactly a pint, so the Guinness head means you're getting less than a pint of actual beer.

This is tolerated/expected and so de facto correct but de jure perhaps not.