Several users reported that their voice stopped working in Google Navigation. The reason was a mismatch between the voice you’ve purchased and the default language on your Android. Update 2.0.6 of the Classic engine fixes this problem. The UK English voice will be used even if the language setting on your device is US English, and vice versa.
Alternatively if you use the UK English voice you can go to Android Settings -> Language & keyboard -> Select language and select English (United Kingdom) .
SVOX Mobile Voices 
Hi,
I have just bought the SVOX Dutch and running 2.2 on the DesireHD. Only problem is that i have my phone’s default language in english. In google navigator i will be prompt to install a voice synth which i skip and after that i hear a different voice than the standard pico tts, in english. Is it possible to hear the dutch voice (which i bought) without changing the phone’s default language in dutch?
Unfortunately not 😦
I was wondering about this myself. I would love another foreign accent on my Google Nav (I’m American but use the English GBR).
If there an update which will allow this, please let me know
I actually ended up rooting the phone and adding the GBR locale manually, then selected it under the usual “locales” menu. I can provide more details if needed.
More details welcome. It should be able to select the en-GB voice in Google Nav if your locale is another variant of English like en-US.