O2 in the U.K. has announced that it will only be selling the iPhone 4 to existing O2 customers at launch. “iPhone stock will be extremely limited in the UK at launch and not everybody who wants one will be able to get one straight away,” the company says on its website. “We want to make sure that our existing customers get priority so until at least the end of July, only existing O2 customers will be able to get an iPhone 4 from…