given your friends info, I would say that Microsoft has not opted to make a uk english windows xp beyond the localized options in the control panel regional settings.
I checked in some international newgroups and nothing contradicts that information
I think you are trying for the impossible here, short of hacking the code. I used resource hacker to search for Minimize and Maximize but was not able to find they so they are not stored in any known DLL (they are stored in a dll or ocx, of that much I am sure--but which one, I cannot say).
I'm afraid unless someone happens to know what resource this is stored in, we cannot help you with this issue