Thanks to a tip from Buz, I found on GBMINI's website details on how to get the British Voice. It requires editing the ISO file that you can download from the BMW x5 site.
You still need the latest version of Nero. My previous version expired, so I downloaded the latest one and it works fine again.
You also need
UltraISO.
The surgery needed is very simple. There are 2 folders that contain the American Female voice. You simply need to swap the American Female voice with the British Female voice by just renaming the British Female voice as the name of the American Female voice. You'll have to rename the existing American Female file so it doesn't conflict. I would just change the extension to BAK.
So navigate to the
V2/Voices/0502
rename usa_f.csf to usa_f.bak
rename gb_f.csf to usa_f.csf
V2/language/BMWOCN
rename usa4f.tsf to usa4f.bak
rename gb4f.tsf to usa4f.tsf
Now make sure your Nero is installed and save the new ISO image, and BURN to CD (all from UltraISO--which uses Nero's engine)
Take that CD and put it in your NAV and let it update the software.
Note, I have a 2003 MC, and it works! The Settings will still show USA--not GB, but the Female voice being used will be British Female. =)
Thanks to GBMINI (Ian Cull) and Peter P. Geremia for the tip!!
What does the voice sound like??
http://www.minisontop.com/oddstuff/English.wav
Mk III 2002 Nav CD Based---You may have to change the voice files found in the ISO under MKIII. I haven't confirmed this, but based on the ISO structure, it appears that changing the files above may have NO effect on your cd based system since it is using different update files.