Sadly for you, Apple has loads of stock on Regent Street. I was there at 8:30 last night and they had been through a disaster of a day, hand-filling forms and running Windows on every machine. They had to send home about 300 people when O2 was preparing to close for the day as well.
For you, I'd call around, particularly to O2 on Oxford Street and the CPW behind Baker Street on Marylebone Road. They may get stock today and these are rather large shops.