13,000 orders a second? Not quite.....
I can believe 13,000 http requests a second to their web servers, but not 13,000 different individuals trying to order. The fact that the order process had to be restarted if was an error would have made things a great deal worse. Looking at the requests made for each page on the O2 site, there are between 10 and 50 requests made per page (some files will be cached). I can't access the upgrade site any more, but it seems much more likely that a few hundred people a second were trying to order iPhones at a time.
I'm guessing that Mr Shurrock was told "13,000 requests a second" by those responsible for the web servers, and that they accidentally neglected to make clear that this didn't mean individual orders.