I have encountered Spell Checking plugin awkward setup behaviour. It comes down to proper config however - so It is **solved** but it is problematic because there is ambiguous information on how to setup it.
1st as explained in here: https://plugins.geany.org/spellcheck.html
Note: if you are using Enchant 2.0 or later (the library used by the SpellCheck plugin), you need to move the dictionaries into a subfolder called "hunspell" in the directory you created above. To check which Enchant version you are using, check "Help->Debug Messages" in Geany and find the appropriate log message telling about the Enchant version.
Files need to be in the _hunspell_ directory
2nd I've downloaded .dic and .afc but I had to substitute dash for underscore in their filenames
ex:
en-GB.aff -> en_GB.aff en-GB.dic -> en_GB.dic
after that I've put both of these in
C:\Programs\Geany\dict\hunspell
_dict_ -is a newly created folder, for my own convenience
3rd In geany spell checker plugin preferences however I had to point to
C:\Programs\Geany\dict
and NOT to
C:\Programs\Geany\dict\hunspell
in order to spell checker to detect **en_GB**