The UK voice of Siri on the iPhone 4S is the voice of Jon Briggs. He does voiceovers. In fact, he's also the voice on The Weakest Link - not Anne Robinson, the unseen man.
He did a voiceover for a company called Scansoft, which later merged with Nuance, which Apple took over and produced Siri.
“I did a set of recordings with Scansoft five or six years ago, for text-to-speech services,” says Briggs. “Five thousand sentences over three weeks, spoken in a very particular way and only reading flat and even. Then they go away and take all the phonics apart, because I have to be able to read anything you want, even if I’ve never actually recorded all those words.”(Source: The Telegraph)
The result, Briggs says is “as close to human speech as anything that’s out there. It gets everything right, more or less, apart from the inflection.”

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