O2 have updated their iPhone availability page, and it now states:
2.3. We are expecting further stock this week.
2.4. For the forseeable future the only place to purchase iPhone 3G will be in an O2 Retail, Carphone Warehouse or Apple store if you are new to iPhone. Existing iPhone customers looking to take advantage of the special, early upgrade can do so in any O2 or Carphone Warehouse store up until 11th October, 2008.
Previously, O2 had said:
2.3 We are expecting considerably more stock from Friday 25th July onwards.
2.4 Soon after this date we are hoping to open our online upgrade shop for existing customers and online shop for new customers
Obviously, demand is still far outstripping supply, and O2 have not received considerably more stock. Either Apple is not able or not willing to supply the UK with enough phones to satisfy demand. The last update to the iPhone availability tracker indicates that there seem to be plenty of iPhones available in the US, so I would imagine they have stuffed their home channel first, and left the international carriers on a slow drip until manufacturing can catch up.
So no online ordering for the foreseeable future. If you’re still after an iPhone 3G, then your best bet is to phone your local Apple, O2 or CPW to check availbility, then get in the queue.