Incorrect British English (UK) hyphenation in InDesign — Hunspell and Proximity both fail

As you say, British and US word-breaking rules differ. But even in the UK there are differences in word-break conventions: there is no such thing as ‘British English hyphenation’ (as Eugene also notes). For example, the word-break recommendations of Collins and Oxford University Press differ in many places. Collins recommends de-moc-ra-cy, while Oxford has dem-oc-racy. Both are different from your demo-cracy (where did you find that one, by the way?).

I’ve worked for UK academic publishers for years, and the way I dealt with this was to stick with the Proximity dictionary and use exception dictionaries. In my experience that worked well. Those exception dictionaries took a bit of work initially, but soon they covered most word-break rules, and each new job required just a small handful of additions after the first proofread.

In your case, now, all you can do is to fix the word breaks manually. And do add them to the/an exception dictionary. And tell the proofreaders to alert you to word-break problems earlier.

This is an interesting one. I suspect you're dealing with a limitation or inconsistency in the hyphenation dictionaries rather than InDesign incorrectly assigning American English.

British and American English can differ in their preferred hyphenation points, but assigning English (UK) doesn't necessarily guarantee that every break follows a particular British editorial convention.

I’ve tested here and it breaks at the same point for me too.

InDesign does support custom hyphenation exceptions, which can be stored in the document. You can also import word lists, so you wouldn't necessarily need to correct every occurrence manually.

Perhaps a script could potentially help build or apply a publication-specific exception list.

The difficulty is that correcting hyphenation can still alter line breaks and pagination, so some recomposition and proofreading would be unavoidable.