You’re probably under Windows. Under Linux, English variants beyond en_US are provided by a separate “language pack” which must be explicitly installed (like any other language pack). See if such a language pack exists under Windows (I don’t know which file type it might look like) and verify it is installed. Otherwise install it.
Regarding default language for documents, I wouldn’t recommend Tools
>Language
>For …
because this is a manual step and you might forget it in a future document. Rather, go to Tools
>Options
, Language settings
>Language
and define it there. This setting is kept in user profile and will be automatically chosen next time you open LO Writer. This works provided you didn’t already change language setting in paragraph style Default Style. In the latter case, remove your custom setting with button Reset
or set your preferred language. If you set it in Default Style, any change in Tools
>Options
, Language Settings
>Language
will be barred by your modification in Default Style.
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EDIT 2019-07-13 (after your comment)
Default language for documents is set through Tools
>Options
, Language Settings
>Languages
as I previously mentioned. I forgot to mention additional settings in Tools
>Options
, Language Settings
>Writing Aids
: this is where you enable the dictionaries. Check that en_GB is in the list.
As you say misspellings are always possible. Eventually, make a list of them and report as a bug.