Have you tried downloading the extra voices? My view is that OSX offers nice British English voices and a South African female voices. All these these are free. Go to system>dictation & speech and try to change the system voice. I used AT&T in windows in past years but not recently so I cant say that they are the same as what AT&T currently has to offer.
If you have alot to read, have a look at one of the answers to one of my questions. Mac OSX Text to Speech Batch - I split a large file into about 300 text files and will need more. I want each one read out in a different voice, to make it sound more like a radio show... and make me realise that there is a new topic/page.
Good luck with your reading!