Unknown to most users, the default dictionary in OSX is "English". Note that this isn't "British English" (which isn't even enabled). To fix this:
- Open system preferences.
- Navigate to Language & Text.
- Click "Edit List...".
- Scroll down until you find British English and check the box to the left.
You will need to logout and back in to make this change stick to any services currently running, however opening an application fresh, like Mail or Pages will cause the new dictionary to be used.