I believe I've made some progress on this. Typical for that to come after a lot of effort in other directions then posting on a forum... (Sigh.)
There seems to be an assumption by LyX, or at least my installation, that the default language is 'English'—not 'English (UK)', etc., plain 'English'—whereas the OS is set to use an English variant. You can see this more clearly if you try use the Thesaurus. 'English' is offered but the pulldown list indicates it's not present, whereas 'English (UK)' is available to be used. On systems that are not set with this as the default system version of English, the spellchecker "decides" it hasn't got a dictionary. The then bails out without letting the user have the chance to try one of the other English variants. (It complains about reaching the end of the document, then disappears.) Manually setting the *system* (OS X) language to "prefer" English then the variants temporarily resolves this. But that's not suitable if you want British English first. A work-around seems to be to select the document language to be what you want (e.g. English (UK)) and select that to become LyX's default document language. A "proper" solution, AFAICT, would be to have LyX realise that some systems will not have English as a preferred language and correctly check what is in fact the system language in use. It's made more confusing, I think, by the LyX Preferences not having a setting for this - they only set the user interface language; the documents/spellchecking language seems to be separate, but that's not clear to users and the document language setting is a bit "buried", so many users won't be aware that there. (Another, hopefully unrelated, confusion is that I have a system Preference Pane for Spelling that something has installed (ha) that says it can't run on Intel-based systems...! Perhaps that "arrived" as a consequence of installing Hunspell while I was trying to figure this out.)
