Last night, anxious to try the new UK voice I found the only way to change from Kendra to Amy was to freeze Kendra w/Titantium. The only problem I encountered is that I couldn't figure out how to switch between Svox and Ivona on the PE the Thrive was easy with Settings, but Settings on the PE didn't have that option under Language & Keyboard, which doesn't matter to me because I MUCH prefer the Ivona voice. I also used Ivona's US voice with all four readers. I used Svox w/CoolReader, MoonReader, FBReader and Mantano alternating between the male and female voice.
I liked the Kendra voice so much that just yesterday I downloaded their UK voice (Amy) and like it very much as well.just hoping for a male option from Ivona. I'd been using Svox's Grace and Michael voices alternately, those having replaced the Pico, but ten days ago I stumbled onto Ivona it has replaced the others on both devices.
Don't have any suggestions per se, but I can speak to my experience (which is very limited because I've only 'discovered' TTS this month).