Might try Apple's metrocentre shop at the weekend although I doubt they'll have any, it's one of their busiest shops.
That is quite a large number. Are they really expecting so many to have problems?
What's the chances of me turning up at Apple Store Covent Garden and being able to buy an iPhone this weekend?
Depends on time and which model.
I would guess if you want the 128G 6+ in Gold...you might be with a chance turning up at lunch time.