change language

If you go into system preferences, international then under language click edit
scroll down the list and select british english
when you click ok it should go to the top of the lit and your spelling will be correct

 

firefox seems to use its own dictionary, this worked for me

in the address bar, type about:config
in the filter box type in layout
look for layout.spellcheckDefault
right click and modify and change the key to 2

open a new window and go to google.co.uk
type in a uk spelling (centre) and it will underline as red
right click the word and go down to languages
select add dictionary, this will take you to the firefox downloads and let you install a uk dictionary

not sure about word, but hope that helps in firefox