UPDATE: On MacOS Sierra, after you have set your language preference using System Preferences -> Language and Region, you can then go to Text Preferences under Keyboard settings to follow step 3 (and onward) from below.
If like me you have not yet learnt to drop the ‘u’ from ‘colour’, then you might have noticed that Mac OS X Snow Leopard makes it annoyingly difficult to switch the system language preference, especially for spell checking (worse, I suspect the interface for spell check language settings is buggy). To that end, below is a quick outline of the steps I used to switch to British English (click on the small images to see the full version).
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