I've found with my cheap GPS that you can put on the Tomtom fun voices:
1. Download the celebrity TomTom voices from http://tomtom.gps-data-team.com/tomtom_voices.php
2. Extract the OGG files from the file using viftool
3. Copy and backup the british.db file from the SD card onto the PC. Open it with Spracheditor
4. Import the exported tom tom audio ogg files into the different commands shown in Spracheditor and edit some to say 150, 250, etc). Change some files to simply say "ahead" rather than "After 1 km"
5. Save the updated british.db file and copy back onto SD card