Navigation Voice is not changing
If all you want is a British voice, then you don't have to downgrade from Google Maps version 9.32.1, there's another way:


  SETTINGS > LANGUAGE AND INPUT > LANGUAGE > English (United Kingdom)


This changes the language settings for the entire phone, but I haven't noticed any major difference, but I do love having a British girl's voice give me driving directions and the audio quality is almost as good as the Ivona British voice.  

I used to use Ivona but prefer this method now because it doesn't require installing an extra app.

Try it out and see if this helps!  It's easy to go back if you don't like it.  Just change it back to English (United States).

If it doesn't work, then please make sure that:


  TEXT-TO-SPEECH OPTIONS > Google Text-to-speech engine


is selected.  And while you're in that screen, download the speech data for that language:


  GOOGLE TEXT-TO-SPEECH ENGINE "GEAR ICON" > INSTALL VOICE DATA > English (United Kingdom)


What's also cool about this method is that your Google Now girl now has a British accent as well!

Hope this helps! :)