We require a signer of a petition to declare that they are a U.K. resident or a British citizen. They can be living abroad. They also need to have an active email account. We require those who live in the U.K. to provide a valid postal code, which is checked against a Royal Mail database for validity. I guess the question is whether someone from abroad who isn't a British citizen could sign the site. I guess the short answer would be yes, but we do ask that a signer make a self-declaration that they are a British citizen or U.K. resident.
On November 25th, 2014. See this statement in context.