It's not about you: I took a look on amazon.com (US), and what was on offer seemed to be mostly US English dictionaries, with no obvious way to confine the search to UK English, if indeed any such are sold on the US site. One might need to search on amazon.co.uk, get an ISBN for a known-UKE book, and then search that ISBN on the US site.
Anyway, my recollection is that the FM spell dicts are an install-time option. If you have the media or d/l files, you may need to re-install to add a spelling language. Having the files present, and letting FM know they are there, may be two different things.
> How did I know this wouldn't be easy?
Before that, how did you know it was even a problem?
In most installs, switching spell-check languages is "merely" a matter of changing:
Paragragh Designer > Default Font > Language:
for the entire catalog ...
... which can be done in one action for an entire file (but not book), by
> Fantastic as always.
Only because Adobe documentation is appalling as always. As I recall, I learned this method on the Forum.
What FM Help (7.1 or 9) has to say about this is:
"You can change the language used to spell-check text by changing the language assigned to the text."
There's a link for PgfDsgnr and using the DeFont tab for changing the language of local text or the Catalog for that tag, but no real clues on how one might "re-locate" an entire document.
> I'm sure they will have addressed it for FM11...
The HTML Help 11 is already up, and has the same vague statement.
I also downloaded the FM11 PDF (30MB) to see if they at least restored the Index.
Same text for spell check.
The PDF was updated 2012-07-23.
Astonishingly, it is authored in FM8 on Unix .
It is way far from a showcase of how to write a manual in FM.