A.k.a. proper English.
I’m enraged to find this feature after nearly a year of living with my correct spellings being thrown in my face as wrong! Colour, favourite, realise. Arg! I’ve been telling the dictionary to learn them all, step-by-step!
Now, however, the answer has been revealed.
In the application you want to change:
Edit > Spelling > Spelling… and then select “British English” instead of (so-called) “English”
Someone should come up with a law prohibiting it being called English when it’s not really.
Oh, and irony of the day (for Americans)… the word “bastardized” is bastardized! Yay! 
Thanks for that - was tearing my hair out!
Hi Finton,
As an avid Mac user I still find that I’m still very much in the shadow of America and we have to play second fiddle, as in the case sited above concerning the lack of information telling us British users to change our applications (all of them !) to British English.
But hey! We’re in the grip of America anyway, whether we like it or not.
Enjoy!
Also, you have to select a text field for the Spelling menu to be enabled.
Excellent! It’s amazing how long you put up with stuff like this before realising there is a simple fix out there. Cheers!!!!
Thanks.. thats great… bloody americans…
one question tho… what is Australian English??? is there a difference??
Oh my god, thank you, I’ve been writing a thesis and it’s been trying to put “z”s everywhere…
Hmm, why am I seeing the option greyed out when I try to follow the instructions you give… Still tearing hair ;(
oops, sorry, needed to select some text, doh!
There’s an easier way to do this
system preferences> International> language> Edit list > add British English to the list> put it at the top of the language list
this will put the default language of all apps to British English