Ah. Thanks so much Webcat86. I already did my pre-order yesterday but I obviously missed the bit when it told me when actual placing/committing those pre-orders will start on Friday so that's really helpful to have such a definitive answer.
In fairness to me from previous experience of what happened last year I had anticipated the UK store opening for pre-orders while the Apple presentation was still going on so I was doing my pre-order on my iPad yesterday while at the same time half watching the final bit of the Apple presentation so my concentration wasn't entirely on the pre-order process. Not that it makes much difference though I think, there is no race to get a pre-order in as early as possible since it is how quickly you can turn your pre-order into a confirmed order on Friday that determines whether you can get launch-day stock or will have to wait a bit. Just for fun when I upgrade (which is most years) I always try to get my new phone on launch day and so far have been successful every year since my first iPhone 4.