Despite Apple's request that he remain anonymous, British voiceover artist Jon Briggs revealed to The Telegraph that he is the voice of Siri in the UK. Briggs received a call from an Apple PR representative last month after the iPhone 4S was launched, and was told that he should not talk publicly about Siri.
Briggs was told by the PR rep that Apple employees were not authorized to discuss products. He responded by noting that he had recorded over 5,000 sentences over three weeks for a company called Scansoft. That company merged with Nuance, which provides the voices for Siri. As a result, Briggs had never had a contract with Apple, which is why he's gone public.
When Briggs was doing the recordings, he was told to speak a very specific way, reading "flat and even." The recordings were then ripped apart to grab specific phonics to build the voice file that is used in Siri in the UK. Those phonics allow Siri to speak just about any word, even if Briggs hasn't recorded a specific word.
Briggs's Siri persona is also used to create announcements at the Kings Cross railway station in London, and is heard in other applications as well. UK readers may also recognize him as the voice in the BBC quiz show The Weakest Link.