I don't know if its possible to change the actual keys, but when you order a keyboard online you can have it in about 6 different languages.
It would be much cheaper to switch the language in you System Preferences and maybe buy some stickers to put on your keys?![]()
You could always buy the UK model of the Apple Wireless Keyboard if you are really in a pinch to type the UK symbols. Id honestly forget about it.
It is very very difficult to change the keys, but that's not your only problem. THe layout is slightly different, so you'd have to find a way to cut an extra hole in the computer. Just use an American keyboard. 'merica, **** yeah!