Apparently you have the English dictionary installed with Aspell. That dictionary contains American, Canadian, and British dictionaries.
Aspell reads your LANG environment variable to determine which dictionary to use. My LANG variable looks like this:
So, if you change your LANG variable to en_GB.UTF-8, Aspell should use the Brithis spelling.
You could also add your dictionary preference to ~/.aspell, though I'm not sure what the syntax is to make such an entry. You can read more about it here:
http://aspell.net/man-html/The-Options.html#The-Options.