This was bugging me, so I figured it out. If you want a Mac's spell-check to accept both American and British spelling, here's how to do it:
- Go to System Preferences - Keyboard and choose the Text tab.
- Under Spelling, make sure it says "Automatic By Language", then click on "Set up:".
- Under English, make sure "American English" and "British English" (and Australian and Canadian English if you'd like) are all checked/ticked. By default, only one kind of English is checked (American English in the case of my bought-in-Philly MacBook Air).