First off, big thanks to Phil Gyford, this is mainly just a mirror of his excellent guide with a couple of changes. The main annoyance I was having was that the “International” keyboard switcher (the little flag icon up in the top right) would not let me remove the “British” keyboard layout which was installed by default (the one you are probably lumbered with now, right ;). The instructions below get around that problem, but as they involve a bit of screwing around I recommend you back up all you data first, yadda yadda.

1. Open Finder, navigate to /System/Library/
2. Drag the “Keyboard Layouts” folder to your desktop (it will copy, not move)
3. Delete the contents (yes all the bundle files) of the “Keyboard Layouts” folder from/System/Library/Keyboard Layouts (please make sure you have a backup – you will need it in a moment!)
4. Now download the Mac OS X British Keyboard Definition file.
5. Unzip it to /System/Library/Keyboard Layouts – it will now be the only file in there.
6. Open International which is in System Preferences
7. Select the Input Menu Tab, this will show a list of possible keyboard layouts.
8. Select (check) British – Windows 2
9. Deselect (un-check) British (this would have been grey’d out if you had not removed the .bundle files from /System/Library).
10. Close International.
11. Copy all the .bundle files which you backed up on your desktop into /System/Library/Keyboard Layouts.
12. Logout and log back in.
13. Enjoy your new found ability to type #’s in a single keystroke!

To make things a little simpler, here is a screenshot of International with the British – Windows 2 keyboard definition selected (and that pesky British one removed!). Your keyboard profile shouldn’t randomly switch back to the default Mac one anymore.