Had similar.
Desire S on Gingerbread : suddenly got an error message reading memory card
Sat Nav continued to work but reverted to US accent.
Reseated memory card and ran factory reset (picking UK English), wiping memory card in the process.
Tried Navigation with the shipped version of Maps, works fine with British accent
Upgraded Maps to latest via Android Market/Google Play, navigation accent went American again
Repeated the above steps above with the same pattern.
Pico is the only visible installed TTS engine.
Test voice is in British English
Google Translate also reads translated text in British English as expected
Navigation is also misreading some words so says I now leave on "3 road"
Live in the UK