How quickly after iPhone announcements can we buy them in stores?

Hi i'm from the UK, and like many of you guys, i'm planning to sell my iPhone 4S (32GB) and buy the new iPhone. My only problem is, I want to minimise the amount of time I go without an iPhone. So my question is, on average how many days/weeks after the iPhone announcements, is the actual store purchasing date? If its 2 weeks or so, i'd prefer to flog my iPhone on the annoucement date, as oppose to earlier.

Thanks!