I have the same problem though I'm using Tablet PC edition without MUI, just between versions of English. English-US won't go away. This wouldn't be a problem except that it means Word will switch into English-US seemingly at random, messing up spell checking. Also I want to the input language as English-Ireland as it recognizes my handwriting significantly better.

It looks like this is perfectly possible, but whenever I reboot English-US reappears.