O2 have put this statment on their site
iPhone 4 will only be available for 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. 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 us.
Whether you're an existing O2 mobile customer looking to upgrade, or one of our broadband customers looking to move your mobile to O2, our iPhone stock is reserved exclusively for you.
I have to say i'm very impressed with how O2 are handling their limited supply of the highly coveted iphone 4's. I fully expected that it would be a 1st come 1st serve basis with the emphasis on getting new customers but no! The focus is to reward loyalty by giving existing customers priority even if you are just a broadband customer bravo O2.
Read the full statement here